I wouldnt mind having to come back to find new stuff like npcs/quests/hunts/treasures. I don't think it would be that hard to implement since they can reuse some of the stuff that is already there. Maybe see some consequences of how you completed the Act/quests.
I don't see why we would be blocked from coming back to Fort Joy Island since we can teleport anywhere like in DOS1, you can actually complete the island by missing a lot of stuff, blocking you from backtracking would hurt a couple of players.
As for the XP, it should be part of the designed maximum XP by balancing it with other content or give really little XP, maybe even no XP but treasures/money/lore/tools/or other bonus stuff that doesn't break the game.
