This is a list of serials that were considered-then-rejected for series 21:

http://en.wikipedia.org/wiki/List_of..._for_series_21

If an extra story had been slotted into Davison's third and final season, then the final story to broadcast in 1984 would be Davison closer "The Caves Of Androzani" () - instead of Colin Baker opener "The Twin Dilemma" () whose first broadcast ideally should have been delayed until 1985.

The again, there are other stories that were considered-then-rejected for the first and second seasons with Davison, the most notable of which would have to be "Project Zeta Stigma", originally conceived as Davison's debut story, before being replaced by "Castrovalva" (which ended up being the fourth Davison story to be recorded, rather than the first - weird).

Furthermore, apart from William Hartnell making way for Patrick Troughton just two stories into the fourth season (in 1966), it has always been the case (and rightly so) that one Doctor bows out at the end of one season, and the other Doctor does not get a proper introduction until the start of the next season.

My apologies for wasting your time.

It's worth looking at the 5th Doctor Big Finish audio adventures - quite a few of the 'lost stories' have been made. In fact, they've pretty much made all of - what would have been - the 6th Doctor's second season. They are great fun and very revealing.

"The Dark Samurai", "The Place Where All Times Meet" or "The Rogue TARDIS"

from the stories on that list and the stories made, I would have gone for the following:

4: Warriors of the Deep
4: The Dark Samuri
4: Ressurection of the Daleks (Turlough leaves)
6: Warmongers (Tegan leaves)
4: Hexagora (Peri Joins)
4:The Caves of Androzani

But that means doing away with "The Awakening", "Frontios" and "Planet Of Fire"!