A mod that let's you tame mobs and then skin them to customize your new mount with different skins (striped, camo, etc.)Maybe an API that let's other mods add support for custom mobs?
I'll add this to my TODO list as it would make a great excuse to test my JSON system with entities.
That being said if anyone else wants to do this feel free. It will be a while before I even get around to it.