Simm said in an interview that he met Smith and Moffat and was willing to (but he also said that he'll see because it would feel like betraying David, silly him !) so I keep my fingers crossed.

It would be wonderful to have him back just one last time. I want to see him react as well! "Did YOUR regeneration go wrong as well?" *pointedly looks at chin* And Eleven's a different man, so he'd have different reactions!