Well, there are a lot of things to consider, like “what would the release date have been?” and “would this have been made instead of the prequel trilogy?” I think there are some advantages, as having the old cast and crew would rein back a lot of Lucas’ control and possibly prevent his more poorly-received decisions while making the prequel trilogy. Yet, I am a bit skeptical, as the EU was booming at the time thanks to the Thrawn Trilogy and whatnot. Would those stories be included or not?
But let’s say if you want to ignore those, then what would have the story been like? I think it would be more or less what the Sequel Trilogy should have been: focusing on the New Republic and the OT cast (now acting as the mentors) as the new generation of heroes would try to maintain it. Seeing how we won’t have too much of the whole “Skywalker Saga” that I believe Lucas pushed for during the actual prequels, we could have a new cast of characters not related to the old heroes. We can still include the Skywalker-Solo kids and maybe one or two could be part of the main cast, though. As for the villains, it would be easy to use the Imperial Remnant or something akin to it, but I’d try more anti-human aliens and/or a group who wants revenge on the remaining Imperials for the Empire’s oppressive laws on them in the past. If you want, the one leading the group could be a Jedi who was kicked out (either from the old Jedi or Luke’s) or the descendant of one. This can push for the idea of redemption and forgiveness among those who have suffered under evil, as well as following up RotJ with the message that “the father’s sins are not the son’s.”
But that’s just me.