This probably rules out the classic doctors in any potential multi-doctor special. At least anything more than a blink-and-you'll-miss-it cameo. As I first thought if they decide to bring back previous Doctors it'll be Tennant and maybe Eccleston if he's willing.
To be honest, that was my worry when I saw all the classic Doctor's that are appearing in the Big Finish production.
I'd like at least one classic Doctor in the TV anniversary special in a meaningful role, darn it! I don't even care which Doctor it is!