Better (and adjustable) random empire creation. Not just to tackle the spawn bias towards militarist empires or to deactivate certain civics (which can be done with mods), but to make the world feel more alive and to make it more idk immersive. I know this can be circumvented somewhat with pre-scripted empires, but they do not affect empires that form during the game (from a rebellion/split, Great Khan etc). This always bothered me in single player, the nations aren't just randomly generated, they also feel like they are.