I know they aren't going to want to repeat past specials, but I think having another "Five Doctors" would be brilliant, give us Seven up till the Present Doctor all working together, I'd also love each Doctor to have his companion along: Seven with Ace, Eight with Grace, Nine and Rose, Ten with Martha (or Donna), and Eleven with Clara (or whatever her name ends up being).
That would be my ideal lineup, at least.
I think that would be overkill myself, and too much like the crazy and overcrowded Journey's End.
I'd rather just team up Smith with Tennant and maybe one classic Doctor like McCoy (who frankly doesn't look all that different than he did in the '96 movie), and watch those three interact and go on a fun little adventure together.
Leave all the hundreds of companions out of it.