Sounds like the TNG episode "Parallels" where Worf saw a number of alternate universes.
I don't deny that I haven't thought about such a scenario but it was just too mind blowing and would involve too many combinations of situations which are possibly incalculable. For instance if a certain PC didn't die a year ago, could he have changed something in the future, well who knows would he make a different decision and so on and so forth. The scenario just collapsed on the weight of its own complexity.
If you want to play "alternate history" games it may be easier for the PCs to run into the alternate selves from different "univereses" eg. the Mirror Universe, the universe of Yesterday's Enterprise etc. Just my thoughts...
Arise, arise, Riders of Theoden!
Fell deed awake: fire and slaughter!
Spear shall be shaken, shields be splintered,
a sword-day, a red-day, ere the sun rises!
Ride now, ride now! Ride to Gondor!
Theoden King: The Return of the King