Logic.
-Like, you have a governor with 200 militia and 300 garrison troops, but you have to come from the other side of the map to destroy a hideout because they can't handle it?
-You've destroyed a faction 10 times, you have 20 towns and 30 castles, they have 1 castle. But they decide to declare war on you thinking they can beat you?
-No alliances
-In my current playthrough (1st I've won), near the end, there are 2 fiefs owned by enemies, and 1 that was owned by a rebel faction that I am not at war with. I have the compassionate trait. Yet I have no way of asking their leader or the town to join us unless I decide to ransack one of their villages (which will destroy my compassionate trait). I can't even directly declare war on them. I have to attack helpless villagers.
-No long-term peace ability. Overriding your own faction lords to declare peace with an enemy kingdom should mean the peace strongly favors the enemy kingdom and therefore, they wouldn't want to break the peace. Instead within 30 days or less you are usually at war with them. If I had to guess, in my current game, once I became ruler, I probably declared war once or twice shortly after becoming ruler, basically because my fellow lords wanted a war. After that, there was probably about 100 or so (could be a lot more) wars my kingdom fought, always with the enemy kingdom being the one that declared it.
-Factions never disappearing unless you are a homicidal maniac and decide to execute every one of their lords. This is actually the biggest problem. The most logical way to win a grand conquest game is to systematically eliminate each country/empire until there are none left. But you can't do that in this game. I really enjoy the 1st half of the game. I like the style of play (although it could include more immersive things). But once you have taken over about 1/2 the map it becomes a long term grind. While you are taking territories on on side of the map, 5 enemies are taking back territories on the other side of the map.
--Oh. I mentioned this was the first I've won this game. But when I did, there was absolutely no in-game acknowledgement that I controlled the whole map. I did get a Steam achievement, but there was no proclamation in-game on 'congratulations, you are now the supreme ruler of Caladria' or 'congratulations on uniting the whole continent' or maybe a scene where one of your envoys comes up to you on your throne, bows before you, and states with some awe in his tone that you 'now control all villages, towns, and castles in Caladria (although there are still rebel forces disputing this, of course)'.
Lots of good suggestions on this thread, btw. Taleworlds should make it mandatory for their developers to read this threads. If any of them have actually played the game through from start to finish (something I frequently wonder about) they would probably agree with most of these suggestions. And while some might be a bit difficult to implement, I believe most wouldn't provide any major difficulties.