If I don't want to see a multi-Doctor story for the anniversary, what might I want?
Well, if an episode is to really encompass all 50 years of the programme, there are only a couple of constants that can be called upon: The Doctor, the TARDIS, the Daleks and the Companions.
Now a story set entirely, or almost entirely, within the TARDIS has already been done. Inside the Spaceship (aka The Edge of Destruction) in 1964, and The Doctor's Wife in 2011.
The Daleks are coming back in the Autumn anyway - and with representatives of their different eras. (It might be like their own little 50th anniversary story). They have missed only a few years of the programme's broadcast, so what could you really do with them that would be all that special?
No, I would opt for a Companion-themed special. There are representatives still with us, and still working, that cover all the Doctors.
We still have Ian, Susan, Vicki, Steven, Polly, Jamie, Victoria, Zoe, Liz, Jo, Captain Yates, Sergeant Benton, Leela (if she escaped the Time War), Romanas 1 & 2, K9, Tegan, Nyssa, Turlough, Peri, Mel, Ace, Rose, Captain Jack, Martha, Donna & Wilf. (Let's see how Episode 5 turns out to see whether either of the Ponds will be around as well). And of course there will be this (as yet unnamed) one:
A sample of these could be involved, covering the different eras.
On a couple of the DVD commentaries, Peter Purves has said he would have liked to have done a story where the Doctor went back to see how he was getting on running the Savages / Elders planet - and he discovered that he had turned into a tyrant. So, imagine a story where the Doctor revisited some old companions and found that their lives had not been enriched through travelling with him, where his involvement had actually made them worse off. Of course, this would all be some kind of alternate reality - say created by the Dream Lord - to break his spirit - and all would of course turn out right in the end.
This would also get us away from the inevitable timey-wimey sort of story which is normally employed to allow different eras to meet.
That's the sort of special episode I wouldn't mind watching.