Insane OP aside, I'd really have liked to have seen a good "buddy" movie/episode with the Doctor and Master played Simm and Tennant. They'd be able to play all the levels so well! -- hating each other, needing each other, having to work together and trying to kill each other all at once.
I loved their chemistry, and if it weren't for the horrible storyline getting in the way they'd've been so fun to watch. So many good opportunities were lost.