Am I the only one who'd like a more original anniversary story than just a multi-Doctor thing *again*, bringing back past Doctors?
How about a season where each episode features revisiting a past companion, ending up with a finale featuring Ian Chesterton?
To be honest - and its the same with Big Finish - if they don't do it for the 50th, then they never will.
Big Finish haven't done it yet. They've teased, they've had the actors work together for a few scenes, but never done an all out interacting multi-doctor adventure.