I know others have mentioned the idea of Ben exiling himself on Ahch-To at the end of the movie. I believe Pablo Hidalgo said he actually suggested this idea when TROS was being developed. I think the best way someone could construct this ending would be to use shots of Ben standing on the edge of the Death Star ruins, before he talks with Han’s memory, and make it so Ben standing on that rock outcrop where Luke becomes one with the Force.
I think you could also maybe repurpose some footage from Silence (he looks older plus I know a lot of that movie has some cool shore shots and whatnot).
On the other hand, for those wanting Ben to join Rey, I think you could use footage from “The Man Who Killed Don Quixote” (although to be perfectly fair, I actually haven’t seen this movie).