Harrison Ford is the obvious one of the original actors to return, a la Leonard Nimoy, to pass the torch to the younger actors who are going to be playing the brand-name characters.
Ford is a bankable name. Hamill and Fischer aren't. Being bankable compensates for the wrinkles.
I can see them going with a scenario like, Ford as Old Han, reminiscing on his deathbed about the terrible and dramatic events in the long ago past...as a framing device to stories of the younger (recast) characters...
Then you get maximum value of all the brand names: Star Wars, Luke Skywalker, Han Solo, Harrison Ford (who is the only actor who is also a brand and therefore valuable to Disney). And you don't scare away the young audience by making them look at a bunch of codgers for two hours.
So we know from the start that, whatever happens, Han doesn't die. That's okay if he's the only one whose fate is known. Maybe everyone else croaks in the course of the three movies, or go dark side or whatever.