What becomes of Vader? Is he imprisoned or does Luke manage to get him free (via smuggling him out or pulling privilege)? What of Leia’s reaction?
What about him (Vader) resuming his life as Anakin Skywalker? From the looks of it, it was a tightly guarded secret in the Empire. It wouldn't be a lie to say he was gravely injured during the Purge, and he only returned after discovering that he had a son. With his scars, I doubt anyone would push too hard for more details, but he would have to find a way to survive without the suit, or something heavily modified to be observably different from Vader's set up. It might even be a nice way of having Luke come to some new understanding of Obi-Wan's "certain point of view" rationalization. As for Leia, she'd definitely confront him, and it could create a temporary distance between her and Luke as she tries to reconcile the fact that "Anakin" survives, when so many good people she's known haven't. Bail comes to mind immediately. And this exchange just popped into my head (not Bail related):
Leia: "Darth Vader."
Anakin: "That name no longer holds any meaning for me."
Leia: "It doesn't? Really? Well, it still holds quite a bit of meaning for your victims. Just as you were never able to fully rid yourself of the spectre of Anakin Skywalker, you can't shed the onus
of having been Darth Vader any more easily than that armor you wear."