I had a weird dream about a mod too, one based in apocolyptic future. Where religions were reskinned as Types, and instead of Catholic, Shia, etc it was cyborg, vampire, zombie, alien-infected, homo sapien, and you'd be automatically allied with all those of the same Type. (Conversion represented altering/fixing them into your Type, still possible on a person or county level) All other alliances would be done as normal. (So yes a zombie ruler could marry a cyborg for an alliance...)
Types would have much, much different events. Religious/Type conversion would be a bit quicker. Many other mechanics would remain the same. (The Cyborg Boss(Pope) has approved your request for invasion but wants to meddle with your investiture... The Master Vampire is dead and now his many kindred children (Muslim) struggle for supremacy! A Zombie Lord (Norse) have called a prepared war, the zombies are gathering!
Cultures would be reskinned as scavenger, luddite, tribal, feral, etc and would be scattered around the map. Both Types and Cultures would have associated traits.
The oceans/seas are toxic, so coastal provinces would have the least holdings but could still be used for quick ship transport. The richest terrain would be in the interiors or by rivers. (as rivers flow outward to the ocean and have clean water)
Army sprites could be reskinned...or kept the exact same. (ie Einstein's quote on post apocalyptic life: I know not what weapons World War 3 will be fought with, but World War 4 will be fought with sticks and stones)
The goal of the mod (if there was such a thing) would be to completely change the map to a single type. For extra inclusiveness, a world map could be used...
Well that's my *dream* mod, with extra cheese on the side!