I know that this will never happen, but here is my dream for the 50th anniversary. The season première is a generic episode involving the 11th Doctor, when, after saving the day with about 10 minutes remaining, the White Guardian appears and tasks the Doctor with teaming up with his previous selves to combat the Black Guardian. Each of the following 10 episodes, starting with the 10th Doctor and ending with the 1st, will have Matt Smith taking part in an adventure alongside that Doctor and his companion(s) in the style of that Doctor's era. The 1st, 2nd, and 3rd Doctors will have already been killed by the Black Guardian and only their companions (Ian/Susan and Jamie/Zoe and Jo/UNIT) will take part in that episode.
That takes up 11 episodes. In the two part finale, the companions are ditched and the 8 Doctors (4 to 11) team up to save all of creation from the Black Guardian.
I'm on board with this myself, I didn't go so far as defining a story, but, I too have envisioned how great I think it would be to have a Series long Arc (bringing in an "Era group" an episode [or 2 groups for a 2 parter?]), culminating in the Anniversary Special
I'd be OK with it being either the Series 8 Climax, or even as the first part of a 3 parter (If there's 2 regular episodes after the Anniversary)
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?