Well, if it's just for fruits mostly, I feel like that could be done differently as well.I know. It's why I'd only suggest it where it is applicable. Like with one of the Spada bases that isn't feasible since most of the npcs are dead and completely replaced by Spada pirates. As for the rewards I'd not edit it too much for it all to keep making sense and not pile more work ontop than necessary but instead apply a difficulty bonus. This is all from the idea to rechallenge old content tho. If the objective is to overtake the current ruler then normal rules should apply imo.
I'd see this more as a way for people who want a very specific fruit to 'bypass' the current holder if it's feasible. And to give more creativity to those that accept the requests.
For example, say Senor Pink survived on Dressrosa and his fruit is still up for grabs.
If someone wants it, they could simply go to a different interesting island, and the host can have Senor Pink be there as well, and come up with a backstory of what Senor Pink has been up to since the defeat of the Doflamingo Pirates, and how he ended up on this new island and what he's doing there.
Sort of like in the canon with Wapol, he ended up somewhere completely new with a new backstory, as opposed to the original island.