Fun and Balance is the most fun and the most balanced way to play Europa Universalis IV.
It's available from my Steam Workshop page or as direct download. Fun and Balance is a mod which tries to make Europa Universalis IV a better version of itself. The mod has very limited goals:
These goals mean that every patch the right thing to do changes, and I need to go through list of fixed I've made and decide if they're still applicable. Many of previous changes I've made become obsolete because most problematic areas are likely to be addressed by future patches.

Playing with this mod shouldn't feel like you're playing a mod, it should feel like you're playing vanilla which finally patched silly things right.

It doesn't try to significantly affect game difficulty - it might increase it slightly by reducing cheesy tactics, or maybe slightly reduce it if you're trying to play naturally.

So here's the full list of changes, ordered roughly by impact, with reasoning behind them explained in detail.

Base diplomatic relations increased from 4 to 8

In vanilla diplomatic slots are always exhausted by every nation. You absolutely need a few strong allies, a few vassals to expand, and that prevents you from having any diplomacy beyond that. All options such as royal marriages, guarantees, marches, local alliances, supporting independence of others etc. tend to get mostly unused as they take precious slots you don't have.

So the mod just doubles base limit, and this opens a new world of diplomacy.

This bumps up difficulty, as allies are much more useful defensively than offensively. It leads to much denser alliance networks, and it's much less likely to be able to get a free attack on unprotected minor. At least as long as you play with Cossacks DLC enabled.

Mercenary limit reduced to half

Both players and AI have access to infinite manpower pool of mercenaries, and close to unlimited very cheap loans.

This means defeats have minor consequences. It doesn't matter than you just killed every military age man in a country, they'll just spam loans and mercs next day. Attrition, manpower buildings, manpower bonuses, all of that matters a lot less when you can just loan and merc.

Unfortunately it's a bad idea to do any drastic mercenary or loan nerfs, as AI often loses all its manpower to self-inflicted stupidity such as parking a doomstack in a low supply province during peace time.

With some testing I found out that the best balance can be achieved by halving merc limit (both base and increase from force limit), which is unreasonably high.

This doesn't affect reasonable use mercs to supplement your manpower. You can even go vanilla style merc spam if you stack available mercenaries (administrative, quantity, aristocratic) and force limit (quantity, offensive) bonuses. AI can handle it reasonably well.

Small AI countries are mostly unaffected as mercenary limit was already higher than their force limit, but for big ones like Ottomans or Ming you can now actually defeat their army and not have army just as big show up next day.

Fort upkeep reduced to half

Forts are unreasonably expensive. Most good players just delete all or nearly all of their forts, and AIs tend to suffer from having far too many crappy forts in wrong places breaking their economy.

Reducing upkeep to half of vanilla values makes keeping or even building forts more reasonable option for the player, and helps AI economies.

Unfortunately there's no way to mod in more understandable zone of control system.

AI cheats reduction

Call for Peace and naval attrition are removed, as they're mechanics which are exclusively applied against the player. AI no longer gets extra free leader.

It doesn't make a huge difference in terms of difficulty, it's just better if everyone plays fair.

Tweaked subject settings to match wider diplomacy

Vassal annexation minimum year increased to 20 years.

Vassal annexation is just half the vanilla cost - it's unreasonably expensive considering you have to pay full core cost for something which will likely be just a territorial core.

Liberty desire from development a bit lower, and vassals don't count your marches strength in their calculations.

Colonies actually care about relative power of themselves and their supporters, but have negative base LD to balance that out.

Diplovassalization max cap increased, but penalty from their development is still quadratic.

Big tributaries care a bit more rebellious.

There's now -100 cap from annexed vassal opinion, so you're not going to accidentally stack it too high by poor annexation timing.

Liberty desire from historical friend or rival toned down.

All of this generally works well for player and AI, and doesn't require unnatural play.

More building slots

Building slots in vanilla are very restrictive, so many building see zero play, and especially AI wastes its slots on useless buildings a lot. The mod increases extra slot from +1 every 10 development to +1 every 5 development.

Improve awful idea groups

Some idea groups are better than others, and it's totally reasonable but two are so ridiculously useless people only ever take them as a joke.

So mod gives maritime ideas +50% light ship trade power and +1 merchant - so you can actually do some trading - so you can actually get some trading; and it gives naval ideas +1 free leader - so you can hire that admiral without taking a general slot from your armies.

This should hopefully move them from joke tier to situational tier.

You can convert in territories

This is controversial change in 1.26 patch. It's not a completely bad idea, as conversion was really fast and really easy - but until some outs are added (like religious ideas giving you ability to convert in territories, or replacement of outright ban with just slower speed), it needs to go.

Everybody can claim states

This feature is locked to Russian Tsardom government in vanilla, but there's no good reason for it, so I just made it available to everyone.

I thought about letting any empire-tier government do it, but game didn't like this idea (without creating a lot of government types), and there's really little downside to just giving everyone this feature.

Rival and Power Projection changes

I'd love to be able to restore rival system from early patches where everyone could rival everyone else - or at least for every great power to be able to rival every other great power.

Unfortunately that's not moddable, and it's very common that you get nobody to rival late game (and therefore very little power projection), or extremely limited choice of countries to rival early game.

The mod therefore increases power projection for great power status, for eclipsing rival, and slows down decay from actions against rivals. It also increases max rival range slightly, so early game you have more choices.

Religious Shift Decision

You can now freely switch religion to one of your capital at cost of some stability. It's disabled for Papal States, as that messes up with the game.

Disable End Game Tag checks for player

End game tag checks are an egregious case of stopping people from playing the way they want for no good reason.

I left these checks in place for AI to avoid checking them one by one if they make sense, but they're disabled completely for the player.

More formable countries

It's fairly arbitrary which countries are formable and which aren't. If you want to become Norway or Portugal and managed to shift the culture (which is admittedly very easy), why shouldn't that be possible?

Right now it keeps your original missions. It probably should ask if you want old or new missions with popup similar to one for ideas.

All those decision follow similar pattern - you need to have fully unified that culture, be big enough, and have admin tech 10. Excluded from this is anyone with existing form nation decision. Also excluded are Japanese, Russian, and Chinese culture groups, as they already have different mechanic for tag progression (forming Japan, Russia, and becoming emperor of China).

Coalition CB changes

Games sometimes need "invisible wall" style mechanics - fairly brutal means to limit where player can go. They don't have to be fun, but they should be very difficult to trigger accidentally. Normal in game mechanic, even negative ones, should be enjoyable.

Coalition system in EU4 fails this completely. It's really easy to trigger - so easy that AI minors in the HRE often have coalitions against them by 1450s - and it's completely miserable.

Most experienced players learn to play around coalition system by juggling truces, or attacking any country which joins coalition day one, or by using cheesy tactics like offering ally land as soon as possible once coalition war triggers (or in previous patches, offering 10000 gold).

If you actually try to fight and win coalition war, game makes it miserable. You can't separate white peace anyone even if you 100% them, so you'll have to keep going back then to swat their rebels. If you had any allies, they'll peace out leaving you with -40% warscore from battles and -25% ticking warscore - which somehow still counts against you even after they leave the war (that should seriously be fixed regardless of coalition issues). After a few tries everyone learns to never even attempt this unfun fight as just cheese it.

Not to mention just how ahistorical and immersion breaking it all is.

We have somewhat limited possibilities to mod our way around it. We could try to rebalance AE and tone it down a bit so you're less likely to hit the invisible wall. Or we could make fighting coalition more enjoyable.

A small modification of changing coalition CB from superiority in battles to defending capital goes very far towards making it a regular challenging war. That's how it used to be in early patches.

Burgundy event chain removed

EU4 is a sandbox game, and it's more fun when different outcomes happen in different campaigns. It's unfortunately been leaning towards very heavy railroading - Ottomans become second GP after player in nearly every campaign, England forms Great Britain, Muscovy forms Russia, Castile forms Spain, all almost every time unless player stops that, Ming never collapses or expands much etc.

It would be nice if we could make things more dynamic, so sometimes Aq Qoyunlu grows into the Middle Eastern menace, or Scotland sometimes won the British struggle, or some German minor seriously attempted unification. Unfortunately there's no straightforward system for modding in this kind of unpredictability without huge gameplay changes.

One piece of such historical railroading is going way too far - partition of Bugundy event chain. It's a totally nonsensical system where major European country gets divided completely disregarding its situation. The mod just kills it with fire.

I'm open to suggestions how to create higher diversity of outcomes.

Defender Aggressive Expansion discount increased

Being defender in EU4 sucks, as you can't use any of your CBs (during the war, or for duration of truce afterwards), can't declare anyone cobeligerent, and don't get CK2-style reparation for winning.

To make this slightly less miserable, mod increases defender AE discount from 25% to 50%. Remember that extra AE for non-cobeligerent attackers still applies.

Rebalanced Religious Conversion Rates

It's really silly that it's easier to turn a Catholic into a Sikh than turn an Sunni into a Shia.

The mod rebalances completely arbitrary conversion penalties (+4 against pagans, +2 normally, +1 or +0 sometimes) into consistent +4 against pagans, +2 against heretics, +1 against heathens.

Trade Map Tweaks

EU4 doesn't support dynamic trade map, and doesn't allow cycles, so any trade map will require compromises.

The mod adds Panama to Mexico and Patagonia to Lima link, and (to prevent cycles) removes Philippines to Panama and Mexico to Panama links.

This lets Asian powers enjoy New World trade from at least Pacific parts of the New World. The tiny downside is that Spain can't transfar trade from Philippines through Mexico into Europe, which historically happened, but that never happens in game anyway.

I'm considering much more aggressive changes, something like Better Tradenodes and Tradeflows mod, but it would require a bit more testing first.

You can use subject's religious CBs

You can declare religious war on your enemies which you only neighbour through subject.

For this both you and your subject need to have religious, and you must have same religious group as your subject (for holy war), or same religion (for cleansing of heresy).

This is mostly to reduce bordergore required to maintain CBs.

Doubled tradition gain from battles

EU4 made a strange change at some point that nerfed tradition gain from actual fighting to very low values, and it became based more on idea group choice than actual fighting. With numbers (land and naval) both doubled, it's a bit less silly, and if you're constantly fighting you should now have decent tradition level.

Religious Leagues as any Christian religion

In unlikely event that HRE will be split between some other denominations than Catholic / Protestant, a league war can happen in a different way. So you could have Reformed, Orthodox, Coptic, or Anglican challengers.

First victory by the challengers will just flip the dominant religion, you need second victory to lock the new religion.

No war exhaustion reduction while still at war

This button is only available for countries at peace, so you can actually make other countries suffer, or be forced to suffer yourself. It's no longer just diplomatic monarch point cost.

To match this, AI willingness to peace out based on War Exhaustion doubled.

Corruption Slider goes twice as far

If you're interested in rooting out twice as much corruption, at twice the cost, you can move the slider all the way to root out -2 a year. This is a necessary change to deal with corruption from too many territories.

It's obviously very expensive to go that far.

China nerf

Rebels in EU4 are too weak, and we all love seeing a Mingsplosion every now and then. So unrest from zero mandate doubled from 5 to 10.

Custom Nations Improvements

Most custom ideas get levels all the way up to 10 at higher cost. There's no penalty for taking too many of same kind of ideas - there's no power level reason for it.

Base monarch stats changed from weird 2/2/2 to 3/3/3 which actually matches average in-game monarch. Limit on distance between provinces of your nation increased to more reasonable values (so you can recreate something like in-game Genoa).

Merchant republics 20 statified province limit removed

It's a pointless nerf to a weak and unique government type.

Also for people without Dharma, Adopt Plutocratic Administration decision has province limit lifted. It's only useful for roleplaying anyway.

Longer CB on Backstabbers

EU4 has CB against allies who betrayed you which last 3 years, so you can only use it if you're willing to truce break. It's as pointless as things get, a relic from times when breaking alliance didn't create truce. The mod increases it to 10 years, but it's a very weak CB, so it's probably just there for roleplaying.

Some arbitrary decisions limits removed

A few decisions like moving capital to Constantinople and recreating Byzantium have a bit less strict limits, so you can use them even if you're doing something unusual like Serbian culture Coptic Ottomans.

Imperial Ban CB

The game has CB to take provinces from non-HRE owners, but it takes 100% AE with HRE penalty, so it's basically useless. Changed it to just 25% AE penalty, for affected provinces only.

Faster peace out

AI willingness to fight a losing war just because it's not been going long enough reduced slightly.

Some overly expensive action cost reduced

Moving capital, moving trade port, and culture conversion are mostly useful for doing weird things, and are quite overcosted, so all of their costs halved.

Everything is optional

As much as possible I tried to make every change optional, and keep it in separate files, unfortunately it's not always possible. It should be fairly compatible with most minor mods. For some popular total big mods (Extended Timeline, 1356) I can just offer separate builds.

There even used to be in-game menu for some of that, but people had performance complaints, so I got rid of it just to be sure.

I'm quite ruthless at killing off any feature which is no longer necessary. If there's balancing issue with relatively simple fix, I'm happy to include it in future versions.

If you think some of the changes cause more problems than they solve, definitely tell me about it too.