I can speak about singleplayer only (never been a part of serious MP game, I'm too damn nervous for that crap) AND as a man who always played EU 2 and now 4 to get immersed in history and stuff, but it all seems to lead towards the general problem of conflicts always (or for the most part) turning into total wars. You declare war for conquest of [insert province] but nothing stops you or another side from all-out invasion WW1 style. Before 1.13, maybe even 1.12 patch when Austria and France used to rival each other most of the time, their first war in 1460s/70s would always end either with HRE Alliance burning Paris or French dancing in Vienna, even if war was declared for some OPM or one province on HRE-France border.
Everyone would be better off signing peace few years earlier without bringing or being a subject to annihilation since their goals were achieved anyway. Conquest of Cambrai ought to remain Conquest of Cambrai, it might evolve into conquest of Wallonia or even entire Austrian Netherlands - true, but is there really a necessity for total carnage that in the end leaves both sides bled out and unable to do a thing for next decade?
Frankly it's not going to happen until EU V is out maybe, but warring system has barely changed comparing even to EU2. From a historical standpoint, barely any wars of that timeline (at least until later) were a massive total wars. Many conflicts had breaks, and could continue for decades. Borders would change back and forth the entire time. From a gameplay pov, oftentimes you won't be even able to achieve your goals without total decimation of your enemy. How many times do AI enemies hang on refusing to peace out since they think their allies who won't or can't help them will come to their aid? In a long-term wars can wreck both defenders fighting for no good reason, and attackers who simply have to go after them because reasons.
What I mean is, no country player or AI, unless they're fighting for their survival, should be forced to fight total wars for every single province or goal or whatever. Silly war declared for one province between major A and major B can lead to armageddon. I have that issue in really chilly Castile -> Spain game at the moment. France keeps sneaking around my possessions in Indonesia or Africa with random islands and single provinces. But purely colonial war would have to end in a totaler krieg of all times with more than 100k of soldiers on both sides fighting in Europe and to achieve my goal (few small isles in Indonesia, Fernando Po, Galapagos, Tahiti) I would have to break through French army, several forts and probably reach Paris. Is it honestly worth it, especially that all I need these isles for is
nice borders? Maybe it's colonial games tiring me so fast (despite me loving them

) but it's not worth the hassle and I just kinda don't fight France even if I could. Random blobs and total wars are not in my interests, at least not in this campaign. Alas, I have no other choice.
Just get rid of the total wars, you know what I mean. That's the issue, that breaks immersion historically, can get annoying inside the game and even lead to crazy situations like one mentioned in post #1. Just stopping mad AI would go a long way. I love the ferocity of AI allies helping in wars, like Venice using its entire fleet and army to beat down Irish OPM, or Circassian adventures to siege down Rousillon/Rosello. Whyyyyyyyyyy. Who needs that. Sending few ships or regiments, fine, but all of them?
I hate proposing new mechanics since we never know what can be done, but if there was a possibility, I would:
1.) Introduce cease-fires during wars. Whoever owns the wargoal province/provinces, keeps them as owned-contested, as in owns them in practice, but his actions are limited. If ceasefire lasts long enough, war ends, provinces go to their current owner.
*Also attacker should be able to add more wargoals during the war. Initially local conflict can evolve into something bigger now, but not necessarily end up in another WW0 just for the sake of few provinces.
*Also, sieging provinces outside of wargoal scope (for example attacking Vienna when having CB in Netherlands) should generate AE or something.
2.) Get rid of 5-15 truces and make them part of peace demand. Five year truce might be the base truce. You can add more via peace demands. Instead of being forced to take provinces to weaken your rival, enforce very lengthy truce on them.
3.) Create a limit of regiments allowed to use in certain war. So if one country attacks another war does not turn into instant total war. Limit wouldn't be enforced, you can go over it, but using more forces would generate more AE in peace deal. Look, everyone around would be scared if France used all of its regiments to invade Netherlands. Who wouldn't be scared of new neighbour walking around with huge stacks, ready to throw them all into meat grinder?
*That would also help with stupid crap like Poland-Lithuania flooding HRE because their HRE OPM ally declared war against another OPM. With limited involvement powerful friends suddenly aren't game-breaker.
*Past certain point, for example when defender can be vassalized in peace deal, he can use 100% of its army, since, you know, he's fighting for his survival, at last.
This is all pure fantasy considering EU IV is already two years old and I imagine PDX has plans for another year or two. Suggestions above are definitely not perfect, but they would tackle heart of the problem - limitless world war zeros, stubborn defenders and attackers, countries annihilated for no good reason.
But then we have simple fun factor and the fact that EU4 is a game after all. More restrictions and convoluted mechanics might mean less fun. After all EU originated from a board game. Maybe overcomplicating matters won't be the best choice. Maybe I don't play the game to play the game but to rewrite goddamn history, so for MP players or players in general all of it might sound wrong.