50/50 is the greatest Big Finish audio never made. It was going to be Steven Hall's third story, following up both the "Black and White TARDISes" arc from the Main Range and To the Death with a character drama even bigger than A Death in the Family. The plot would have pitted the Seventh Doctor against the Eighth - McCoy, in full chessmaster mode, looks into his future, decides that McGann isn't up to the task of being Time's Champion, and decides never to regenerate. In response to this, McGann punches a lot of holes through time and reality, letting all the Elder Gods the Seventh Doctor had been fighting up to that point through in an attempt to kill Seven and ensure his own existence. Both Doctors' companions would have been caught in the crossfire and forced to choose sides, the twist being that they each ended up with the regeneration they'd never traveled with.
Unfortunately, this magnum opus never came into being, since Big Finish couldn't scrape together a budget for all the actors it would require, and they ended up making Gods and Monsters instead. BRAVO BRIGGS.