Were I on Larian's shoes I would make Tav some sort of exiled planer regardless of race. If I remember correctly, all the races have representation in sigil, so just make small background about the MC doing something to piss off the lady of pain, but leave it vauge. Since you're not going back to sigil in this game you are basically a clean slate.
I like the idea, but she would send the MC to the maze as punishment.
