I think something nice would be for Rey to end the story with some kind of boon. New knowledge or wisdom that will keep the new Jedi for repeating the same mistakes. But nothing is ever really brought up clearly regarding that, at least in TROS.
Yeah, that would certainly be nice. If JJ actually cared about building on TLJ, he probably would have had the main lesson be something along the lines of “Injustice is wrong, but hesitating to fight injustice because of an outdated dogma is just as wrong.” Trevorrow’s Duel of the Fates script went too far with the whole “Grey Jedi” thing, but I think he was on the right track. However, JJ didn’t really care about building on TLJ, so he just ended on the exact same message ROTJ ended on. It would be pretty hard to change that in a fanedit though.