I really like this idea.

Part of me also wants it for the main questline and maybe guild quests.

At the very least a rematch with the final main questline boss would be fun. Especially if it was somehow made into a challenging group battle in some way shape or form in the future (despite the logical problems that idea naturally presents.)

The way it could be implemented could be similar to Cadwell's Silver and Gold. Hypothetical resettable zones could perhaps be a further reward of those two completed, that you could activate at your discretion.

Realistically though, they'd probably rather wanna focus on new content and quests (and you can always just make a new character if you wanna re-experience quest-lines.)