What If?

This is an idea I had this morning, so I haven't thought out all the angles of how to operationalize it best. Comic books have traditional had stories where things they didn't want to institute in the primary continuity occurred: DC called them imaginary stories; Marvel placed them in the pages of What If?

I've usually run superhero rpg campaigns just like most rpgs. The past is immutable and a bad outcome for the PCs is a bad outcome. What if one borrowed a from What If? You could give the PCs a "retcon" session, beginning perhaps at the point of one pivotal change in events. After the retcon session, the group could decide which continuity the campaign would continue in. The other wouldn't necessarily cease to exist, but could be the sort of visitors from alternate timelines to interact with the PCs later.