Blades In The Dark

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 😅

I would go the Scum and Villainy way. If you play a 'monster' race, replace your starting ability with a 'Xeno' one. Player and GM should work on it together. Quote from the rules:

If you want to play a xeno with unique abilities, replace your starting ability with the following: “Xeno: You may spend stress (0-2) to perform an inhuman feat only members of your species can do.”

Reflavouring is always an easy and safe method. Someone else have already done the balancing. And if you don't faind exaclty what you are looking for you have examples to work off.

That being said... For example getting into the baron's mansion undetected through the third story balcony is the same conflict regardless if you are flying or climbing the walls like a gecko. You have position and effect for balancing. Flying may be Desperate (easy to spot/shoot down) and Great for example.