BFP had 8 stories of Doctor Who Unbound, which were alternate ideas
What if the Doctor had never left Gallifrey?
What if the Doctor was not the Renegade Timelord imprisioned on 20th century earth?
What if The Doctor committed a totally immoral crime?
What if the Valeyard had been the one to escape from the Matrix after the trial?
And, finally, the relevant point: What if the Doctor had regenerated into a woman. This was done as a comedy, and is the worst product I've run into of the entire Big Finish line. The Female doctor, to hide from the Timelords, works at a Tescos grocery, gets drunk, and vomits. Repeatedly. Repeatedly. And once again. The sonic screwdriver is used to help pensioners get their coins out of jammed trolley cart locks.
That said, I like the idea of the unstable regeneration running through multiple actors/bodies. After all, Romana got to try several possibilities.