"what will we do for *this* anniversary?"
"Same thing we always do - a multi-Doctor show"
Provided they can come up with a new take on it, or at least an interesting story, why not take advantage of the audio medium and do something that just can't be done on screen now with the same effectiveness? It can also be aimed more squarely at the fans without having to take the casual viewers into account.
The TV series may do something entirely different.