There's a somewhat depressing quote from Sylvester McCoy (especially if you imagine it said in his despairing, plaintive voice) over at i09 from when McCoy was asked whether he'd heard anything about the classic series Doctors coming back for the 50th anniversary;
"No. Nothing. Niet. Zilch. The other day I was with Tom Baker, Peter [Davison], and Colin [Baker] – the 20th century Who-ers, and Tom asked 'Well, little man, have you heard anything?' and I told him 'No, I haven’t heard anything.' So none of us have heard anything."
I'm hoping this is smokescreen or they simply haven't been approached yet. Otherwise, I don't know how I couldn't see Steven Moffat as absolutely heartless. Peter Davison's also quoted as saying he doesn't think it'll happen because the actors' ages aren't right but anyone with a tiny bit of exposure to the show would know there's a million ways of explaining that. Like the miniature episode "Time Crash" from David Tennant's tenure when Peter Davison appeared. I believe his age was explained as some kind of temporal anomaly.
Okay, I have been thinking about this. For months. Which may be sad. I boiled down different premises I thought of into this;
Open the episode in the Dark Ages with Sylvester McCoy as Merlin. This would provide the backstory to Battlefield, where the seventh Doctor finds evidence that he's going to be Merlin at some future date. Since Sylvester McCoy doesn't look much older than he did in the 1996 TV movie, there's no reason to explain his age.
We see him hiding or creating some kind of terrible power, then we cut to Paul McGann engaged in the war between the Time Lords and the Daleks. He's forced to unlock this terrible power, which both contributes to the destruction of the Time Lords and causes the Doctor to regenerate into Christopher Eccleston--so that regeneration would finally be on record.
Then finally we get to the eleventh Doctor. In the course of an adventure, he encounters something that injures him by reacting to remnants from that Merlin power. He seems like he might regenerate, but he's gotten some kind of temporal infection that causes his body to reuse old matrices, so he turns into Colin Baker. The age is explained by the fact that it's an old matrix and degraded, in fact it starts to get worse as the Doctor and Clara search for a cure. They decide to seek out the copy of the tenth Doctor living in the other dimension with Rose since, as the immediate predecessor of the eleventh Doctor, the tenth has a clean blueprint of eleven in his DNA.
At some point, Colin Baker "degrades" into Peter Davison and Davison into Tom Baker. And at some point Romana shows up.
I'd like it if the episode ended with Sylvester McCoy showing up again, showing that he'd somehow masterminded everything to some purpose, as the arch puppet master thing seemed to be where McCoy's arc was going, possibly in relation to the Trial of the Timelord Plot. Maybe he put this all together to find a way to prevent the birth of the Valeyard.
In better TV news, I'm really excited about the Veronica Mars movie kickstarter getting funded. Over two million dollars in one day--and it's still going. That show deserves the love, too--it was a wonderfully smart and funny series that died much too soon.