i thought the vassal limit was a good idea, it was just handled very badly and soured everyone on the idea.
charlemagnes empire did. also, the roman empire was still around, we just call it byzantium.
france, england, byzantium, HRE. each of these held together for centuries. and there were many more.
also, i agree that coalitions become localized is a good idea, but doing so would nerf them to such an extent that there would be no point in having them around.
Charlemagne's empire was broken into pieces rather quickly.
England and France were not empires in this period. So called 'Angevin Empire" was the biggest territory ever controlled by England in the Middle Ages. It controlled parts of France, but never entire kingdom. And some people here nonchalantly say "I just conquered the kingdom of Poland"....
HRE didn't really expand dramatically. It reached it's peak when it controlled parts of Italy, and then dwindled.
Byzantine Empire expanded on several occasions during the Middle Ages. But never so dramatically as it did under Justinian. Most of the time it was fighting to retain it's territories. And it dwindled with time, until it was destroyed.
If I play in Russia, as one of the Grand Princes, and form kingdom of Rus, I usually come under threat from Poland, Lithuania or Hungary individually. That threat is not so significant for me, since I believe I'm smart enough to handle AI kingdoms smaller than me. But if those realms combined their power, and joined a coalition against me, I would be worried.
And that's exactly what I want. A challenge.
It is not a challenge to have a Cuman-Norse-Polish coalition. It sounds (and is) ridiculous.
Coalition of Baltic States and Finnic tribes would be a far lesser threat, but still more serious than those realms individually.
Or if I am the Emperor of ERE, and I get a coalition of Serbia, Croatia and Bulgaria. That would give an actual challenge in the Balkan peninsula. It still wouldn't be too hard, but at least it would be interesting. Those countries as of now stand no chance against me. Or maybe a separate coalition of Italian States, Sicily and North Africa. That would make me think twice before expanding in that area.
You say that such coalitions would have no point to exist. Do you still think so?
Gameplay-wise, it would be great. Maybe you could still find arguments that it's not as historical. But really, there are so many mechanics in the game that ignore history, simply for better gameplay.