I posted this as a response in the DVD section, but figured it mau get more views here. This is my idea for the big 50 celebration with all of the surviving actors reprising their roles.
The Doctor is contacted by the Time Lords for help. Knowing that this is impossible, he doubts the calls genuiness, but his curiosity as always, gets the best of him. Upon investigating, he finds that is is in fact genuine Time Lords who made the call. Time Lords from another dimension where they never were wiped from existence. But why contact this Doctor (Our Doctor) from another dimension? Simple, their Doctor is dead, in his dimension he has already gone through all of his regenerations and is no more. Our Doctor, the 11th is the one they contact for help. Having a chance to walk among his people one more time, even though he knows that fixing the problem will set things back the way they were where he is alone, still cannot resist helping them. They explain that multiple universes are collapsing and being wiped out of existence. He needs to find the source and stop it, before all is lost.
They also offer him help! Help in the form of himself. In the now accessible multi-universes, he comes into contact with the alternate versions of himself. In their respective universes, they have not yet regenerated to their next 'doctor'
Tom Baker's Doctor is old, in his universe he did not fall off of the radio tower, he is still the Doctor.....
Peter Davison did not succom to poisioning....
Colin Baker did not crash.....
Sylvester McCoy was not shot....
Etc. etc.
So we see them as they are, old but still the Doctor!
There is however, no timeline where Doctor's 1 through 3 exists anymore!
The Doctors must all work together to restore the universes and send themselves back to their own realities....
Add fluff, filler and a bunch of cool sonic screwdrivers here.....
Fix the problem, send the other Doctors back to their homes here.....
Matt Smith returns to his timeline, smiles and tell his new companion, "In the words of a dear old friend of mine.....Lovely Chaps....all of them!"
So yes, it CAN be done. This is just my idea, Moffet is a lot smarter than I am, he can come up with something!