Usyk has to first deal with the rematch with Joshua, I doubt Joshua will accept some money to step aside as if he did then most likely he will never receive another chance to win a title, so it is all or nothing for Joshua when he fights against Usyk again, however I think that taking into account what we saw it is almost a given Usyk will win the rematch, so what comes next is the biggest question. And while I would like the fight between Fury and Usyk to happen right away, we know that promoters will get in the way.
The biggest problem we need to get rid in boxing is, mandatory rematch clause for one sided beating . My prediction is still Oleksandr Usyk winning the rematch and then meeting Tyson Fury for a mega showdown.
For Usyk, it's better for him to give Joshua the rematch as beating him twice will not make people doubt about his real talent in boxing, it will also boost his ranking among the best heavyweights in the world, and no other opportunity for him after winning in a rematch but to fight with Fury who is currently the number 1 in the world. Then probably loser vs loser or (Joshua vs Wilder) too.
Having said that, Joshua also needs to change the habit of loose his first fight with the opponent and later defeating the person in the rematch because it making it look like he's selling the first fight.