Well, when I really stop and think about it, if Simm still wants to be The Master they should probably let him for a bit. With a show like Doctor Who that is designed to go on indefinitely it would be silly to waste talent like that when it wants to stick around. It would also be great for continuity, to have him return even if for just one episode with Capladi to give more of a link to the past. That's why I'm hoping if this Dance guy is a new Master we will at least get a regeneration scene. I hope I hope I hope.
It would also be kind of an interesting twist as well. When we saw The Master regenerate from Jacobi to Simm he said, "If The Doctor can be young and strong, then so can I," and he regenerates into a younger man to match Tennant. To see him regenerate from a younger man to an older one to match Capaldi would be kind of cool. It would almost be like he's regenerating to match The Doctor.
Or is that too much?