I hope they do a multi-Doctor story, and I hope they do it as a comedy. No Time War, no Omega, no villain who threatens to destroy the universe. That kind of thing can only work if it's done with tongue firmly planted in cheek.
I suddenly had another idea: A comic relief special where the Doctor finds himself at some kind of crossroads where he doesn't know what to do, so he goes inside his own mind to ask advice from his previous incarnations.
Can't imagine Cumberbatch would age into Hartnell. How about Tom Hiddleston sporting his Loki hair?
To be slightly more realistic, I'm thinking
Harry Treadaway or
Douglas Booth.
Harry Treadaway I could see. However, I don't think that Douglas Booth has the right look. I also thought he was awful in that recent version of
Great Expectations.
Personally, I'd go with a Trials and Tribble-ations / Back to the Future Part II style insertion of the current Doctor into past Doctor adventures.
For technical reasons that won't work. The video resolution of the old series would look like crap at high-def, especially when they digitally edited the eleventh Doctor into the proceedings.
The video resolution of the old series does look like crap at hi-def. However, they could easily degrade the new Matt Smith footage to match. I even think this could be a good opening teaser: After a quick cold opener, the TARDIS begins to shudder and the 11th Doctor is knocked to the floor, unconscious. We see a close-up of the Doctor as he and all of his surroundings turn into a low-resolution, black & white haze, ominously leading into the opening credits. (The Doctor could even add some line later about how the temporal distortions make the images hazier the further back he goes in his own timeline.)
Or maybe they could just do a season of episodes that try to match the tone & style of old episodes. Like, they could do a pure historical to represent the Hartnell years, a scientific research station under siege story to represent the Troughton years (perhaps with the Cybermen), a UNIT story for Pertwee, something wacky for the Tom Baker years (perhaps while bringing back the Zygons or the Rutans), etc. (What would be a good style for representing Peter Davison or Colin Baker? All I can really remember from that era is the incessant screeching of Tegan & Peri.)
Another thing I'd like to see is a multi-Doctor special where the Doctor meets a future incarnation, like "The Next Doctor" but for real. Like, I imagine Matt Smith announcing that he wants to leave the show. They decide that his departure will be in the middle of the season instead of at the end. This way, they would have to cast the replacement somewhat in advance. Early in the season (like episode 1 or 3), they show the 11th Doctor meet the 12th Doctor. Then you'd have the regeneration sometime around episode 7. Then, after some other adventures, towards the end of the season, you see the multi-Doctor story again but this time from the 12th Doctor's perspective.