"The Ten Doctors" ...
or the Twelve or Thirteen or Fifteen or however many doctors, if you're a fan of the quasi incarnations played by Cushing, Atkinson, Grant, et al.
Just for fun, taking footage from various episodes to edit every single actor who's played Doctor Who into a single nonsensical episode. 1st Doctor Hartnell meets Rose, in black and white.
Just being silly.
EDIT: Thinking on this some more. I think the largest number of Doctors in a single scene the BBC ever pulled off was three, in The Five Doctors. And Hartnell on video in The Three Doctors sort of counts. A very clever fan could probably do better.
In a trailer for the Tennant series, Tennant morphs into Rose for a second. Some effect like that (it's a simple Photoshop trick) could be done to show the Doctor morphing into his past selves. A new linking plot could be filmed starring some amateur in a costume putting pieces on a chessboard or some crap like that. This is Doctor Who, so with no offense to the BBC, shooting links on a low budget wouldn't really stand out much.
The piece could be kept short ... even done as a trailer first. It would mainly be an experiment to see if you can make it appear that actors filmed in different decades, often on different stock, are acting together in the same scene. Video stock from Doctors 3 through 7 will look okay. Hartnell, and any Troughton not from the reunion eps would require the whole scene to be black and white, which could probably be faked. Eccleston, Piper and Tennant (and oh god, McGann) scenes would require different treatment as they look newer and filmier, but hey. The two might actually go well together, if you treat the hell out of the latter.
You might want to seek out other performances by the Doctor actors, in other films and TV shows, to get some less familiar line readings, for audio at least. For some reason Tom Baker in the recent remake of Randall & Hopkirk Deceased comes to mind, as he's playing a funny mentor.
Yeah, okay, just thinking about it for fun.