I assume Bill's mother will show up at some point, too.
What would be good would be if it was the Doctor trapped in the vault, and what we think is the Doctor is actually the master in disguise.
Still don't see how the John Simm version fits in. If the Master regenerates from Michelle Gomez back to John Simm, is it a degeneration?
Well he was doing his evil Master face in the "this series" trailer.