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.

Goals of the mod are Fun and Balance

The primary goal of the mod is to let you choose your own adventure, and play in whichever way you want. It removes tons of arbitrary restrictions and forced historical railroading, and adds new options not available in vanilla.

The second goal of the mod is to balance things out - players in any game will inherently tend to gravitate towards things which are more powerful, even if they're having less fun this way. By helping many of currently underpowered options the space of viable strategies expands, and players are more likely to find one they like without constantly being pressured by the game to abandon it and doing more powerful thing.

The mod with extreme prejudice doesn't want to do any of:

Technology behind the mod

Fun and Balance also happens to be using the most advanced modding technology out there - nothing whatsoever is done by editing text files, all code is treated as data, which allows many kinds of transformations which would be extremely tedious or close to impossible otherwise.

If you're interested in modding Europa Universalis IV and other Paradox games, you can access all tools I use on github.

Choose your own adventure

Fun and Balance has menu accessible from decision system which allows you to turn some of the more intrusive features on/off to customize your campaign.

Not everything can be modified this way, fortunately changes in Fun and Balance are independent from each other as much as possible, so they should be straightforward to change manually.

New opportunities

Better Diplomacy

Base diplomatic relations limit is doubled, as well as most relations bonuses, so everyone can now engage in interesting diplomacy instead of saving their every last precious slots for necessary vassals and one blob ally, with nothing left over.

Resulting dense network of alliances, marriages, and guarantees makes diplomacy much more challenging, and brute expansion much more difficult.

Bonus for heir chance from number of royal marriages is reduced from +5% to +3% to ensure similar rate of succession crises.

Marches were extremely underpowered, so decision to revoke march now only costs -100 relations for 10 years instead of -200 relations pretty much indefinitely. They are still fairly underpowered, but thanks to higher limit on relations you're not basically wasting precious relation slot forever on some tiny bonuses, so they're somewhat closer to legitimate strategy.

Diploannexations cost only 1/dev (as restoring it to pre-nerf system where they only cost diplomat time is not possible), but annexation is only possible after 20 years of vassalage. Autonomy at diploannexation is set to 50%, same as with conquests. Opinion penalty for annexing vassals is capped at -100.

Diplovassalization's hard cap of 100 development removed, but with quadratic penalty for development nations are very unlikely to accept vassalization unless you're really huge, and even then only for a bit over 100 dev.

Protectorates are completely removed from game.

There are no scaled truces, they're all uniformly 5 years.

AI is somewhat less stubborn about sticking in wars forever, with length of war penalty to willingness reduced by 1/3. Penalty for overly long war is the same.

Minimum time to get 100% warscore for fully occupying war leader decreased from 5 to 2 years. This feature was added as exploit prevention, but 5 year timeout was mostly getting in a way of legitimate strategy and just forced player to come up with other exploits. 2 years is a good balance between giving enemies' allies chance to intervene and avoiding pointlessly prolonging wars years after they're over. 

Better Power Projection

Power projection numbers are mostly reasonable when they happen, but they decay too quickly, making bonuses like declaring war on rival disappear often before the war even ends. Many such bonuses and penalties now decay two times slower.

Bonus for eclipsing your rival increased from 5 to 30. It made no sense that it was basically a penalty (as you lost more in long term rival bonus than you gained from eclipsing bonus), and with no way to stop the game from arbitrarily making the same countries unrivalable and then rivalable again, it's a reasonable workaround.

Rival selection range increased from 150 to 250, giving most countries wider choice of rivals, and no longer forcing you to rival 3 closest similar sized countries regardless of your plans.

Dynamic League Wars in HRE

League Wars were a fun idea, but making Orthodox HRE restoration impossible is unacceptable.

The system got changed to much more dynamic one, where any heretic elector can start league for their chosen heresy (Protestant and Reformed only after 1550, Orthodox and Coptic any any time), which can then proceed to .

Due to limitations of the system, once league for given religion triggers, the only possible outcomes are victory of Catholicism, victory of given religion, or peace of Westphalia - other heresies are locked out. In normal game without player involvement Protestant league will trigger overwhelming majority of the time, with occasional Reformed league instead, but with player involvement or custom setup anything is possible.

The mod returns to EU4 1.8-1.10 system where league victory replaced official religion, but didn't immediately lock it, so multiple league wars are possible.

Minor HRE change is that emperor's CB to take imperial territory from non-members now gives only 10% AE.

Dynamic Mission System

Missions in Fun and Balance are a lot more fun.

Nation Designers Enhancements

Nation Designer now adds missing bonuses, bonus levels up to +10 (at steeply increasing cost, so only really available in easy / very easy mode), missing government types, and increases minimum distance between provinces from 200 to 1000 so you can create nation shaped like Genoa if you want.

It also fixes default monarch/heir stats from 2/2/2 to 3/3/3, which correspond to actual average monarch stats, so custom nations don't suffer from -1/-1/-1 penalty for their first two monarchs.

All penalties for having too many ideas from same category are removed.

Holy Sites System

The mod adds optional (can be turned off from menu) CK2 inspired system of Holy Sites. Most religions get 5 holy sites, and small bonuses if country of their religion holds them, with much largest bonus for the whole set.

Countries can also get missions to conquer holy sites - either ones controlled by their neighbours, or anywhere in the world if they complete religious ideas.
Not all religions in Extended Timeline got holy sites yet.

War Exhaustion is real

War Exhaustion now represents actual exhaustion from the war, and isn't just a meaningless dump for monarch points.

It is not possible to pay to reduce War Exhaustion while at war, AI willingness to peace out based on war exhaustion is significantly higher, and high war exhaustion results in penalties to morale and defensiveness.

EU3 style republican elections

The mod restores EU3 style republican elections, with choice of 3 random candidates, and no stat gain on reelection. Tradition loss due to reelection is lower (scaled -3 instead of -10).

It can be disabled from menu, restoring vanilla system.

Missionary rebalance

It made no sense that it's drastically easier to turn Catholics or Hindus into Sunnis than Ibadis into Sunnis. EU4 had a lot of local missionary bonuses to reduce certain conversions, but they never worked very well.

All of them are thrown away, and replaced by base 2% for converting heretics, and base 1% for converting heathens, and base 4% for converting pagans. This leads to much more sensible and symmetric dynamic at all religious boundaries.

Missionary strength penalty for religious centers is reduced from completely unreasonable -5% to more reasonable -2%.

Subject Religious CBs

You can press Holy War CB and Cleansing of Heresy CB against countries you don't border but your vassal does.
You can only do that if both you and your subject have Deus Vult idea. For Cleansing of Heresy you both need the same religion, for Holy War it's enough that you're in same religious group.

Other Balance Tweaks

Mod versions

Versions of the mod for vanilla and for Shattered Europe have same features.

Version for Extended Timeline has many features disabled as they wouldn't necessarily make sense in every era. It has one extra feature in that after 1900 you can buy manpower for money by decision, as mercenaries are currently broken in that era.

Enjoy the game.