A disguise spell would be really cool. Say you cast it, and you appear as a random race/gender, or even a creature. Ideally you could cast it on a target and only they would see you differently, but if that's not feasable it could work mechanically similar to the polymorph spell, so you appear as something else (without changing any of your stats). ICly, you could think of it as creating a continuous sphere around you, and everyone inside is affected by your disguise spell.
You could also disguise yourself as a type of creature to prevent that type from attacking you (since they see you as one of their own).
Obviously this can be RPed as well, but if the mechanics can support it I could see it being a lot of fun.