Soooooo, hopefully it is still relevant to anyone, since I just found this post and actually have something :D What I would do (and probably will in the future, since I have a setting idea that would involve this), is to take a look at the special rules that exist right now for not-exactly-human player entities: ghosts, hulls and vampires.
I am personally looking at hulls, and how having a different hull size changes starting features, but also the way vampire feeding works… Hope this helps and can’t wait to hear about what you came up with, since the post is two years old hahaha 😅