The Theory: Harrison Ford will have a cameo appearance in the movie via a flashback (or in the context of this movie, flash-forward) taking place shortly before The Force Awakens.
Why It Can't Be True: Harrison Ford already had his victory lap for the Han Solo character, so he's probably totally done with the character, and let's be honest, even a cameo from him would add a few million dollars onto the cost of a production that's already had its share of issues.
Plus, in a movie that's already going to likely be rife with fan service, this could be one step too far.
But..What If?: It would be one hell of a crowd-pleasing surprise, that's for sure, especially if the scene featured Han and Lando just sitting in a bar or something, reminiscing on old times.
It's hard to imagine Disney not at least floating the idea to Ford just because of the boost it would provide a film that's been struggling to make it to screens.
And above all else, it's a sneaky way to give old Han Solo more screen time in the Star Wars universe without cheaply resurrecting him or any such nonsense.