How about a quest whose reward is a resetting of all quests to the beginning. You keep the original rewards, of course, but Pingy will surely be happy to put everyone back to work.
Or, a quest where, when you start it, all your upgrades disappear, and you have to go through a series of mini-quests to regain items, only when you get them back, they are slightly buffed over the originals. So you go for a time without, have to regain them, but the advantage is, of course, over time. Don't know how fun that would be to code, but it seems doable.
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