I would personally like to see an entity folder in the Minecraft->models folder. I've been looking for a way to edit the models of entities, but the only way that shows itself is the Optifinecem method that is very tedious to work with. I would also like to see it formatted like the .json models of normal minecraft.
Another part I think would benefit this idea is if the mcpatcher folder could get a similar entity folder as to change an entities model and texture based in NBT data. I think the player model should be able to be edited too, even if its not much to work with.
I can see specific servers using this idea for maps, minigames, and other types of gamemodes; but also singleplayer maps using this too.