The two sorts of questions - music and history - overlap.

For example: If the French had come out on top in America, what sort of music would we be playing and hearing?

Or: If Tony Blair's student band Ugly Rumours had been a smash hit, how would that have affected subsequent history.

But on the purely musical side maybe one of the most interesting is: What if Huntingdon's Chorea hadn't struck down Woody Guthrie, and he'd still been around all these years?