How AE/coalitions should work:
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Coalition warfare should be the most challenging and exciting prospect of an expansionist EUIV game. Yeah you can more efficiently expand by avoiding giving your enemies huge free bonuses ... but if you want to do it the quick & hard way - enjoy the ride.
1. The ability to make separate peace deal in coalition (with or without penalty) is actually what most players want here. I don't think it's Paradox's intention for coalition mechanic. Coalition is there to prevent many OPMs from getting swallow in one war at once. But I believe Paradox will have to give in at some point.
2.The ability to induce a member out of coalition. I believe this is not an intention behind coalition mechanic either. It's precisely there to prevent you from isolated your target unlike normal alliance. Paradox even make a member joining coalition war disregard truce. So I doubt this will be implement anytime soon. The problem should be solve if Paradox let players do 1. anyway.
3. Bonus for being in coalition. While this sound interesting, I don't know how this will fare in game balance term. It will require a lot more thought into it. I'm quite sure those who allow a coalition to spiral half the world span will soon find themselves fighting against coalition with 20 morale and 250% disciple or something as well. More rage thread will ensue lol.
All in all, your suggestions sound like a nice singleplayer campaign. But that is what it is. A campaign where scenario is scripted and units can pop out of thin air, not a sandbox game.
In reality, If countries gradually join a coalition like that, it will give us no challenge whatsoever. Just an EU3's rolfstomp ride.
Poland+Hungary free alliance won't be enough to stop human OE with Greece. We all know that and Paradox know that.
That is why they tune AE up in 1.2. That is why coalition form sooner in 1.2. In hope AI will gather enough strength to challenge a player. The AIs still fail apparently.
This leave us with a few choices.
1. Make coalition form even sooner and more AIs dogpile on you. Current approach. I doubt it will do us any good. Please don't do it Paradox.
2. Make AI actually challenging in warfare. Ideal approach. But I doubt AI will be near veteran players level anytime soon. Human will always find the way to exploit the shit out of AI.
3. Give AIs absurdly resource advantage, Civilization style. I know many players oppose this. But this is actually most practical approach. As there is players who struggle against 1:1 AI as it is. We need various difficulties.
Your suggestion for giving AI advantage, not game-wide, but on coalition basis is interesting really. However I can't see how it will fit in game balance. Some players seem to be annoyed enough with lucky nation alone. and how will we prevent players from exploiting this benefit.