So many scenarios out there...
Some say Fury should fight Usyk next. Some say Joshua and Wilder are done. No one knows what Fury will do next. Obviously, no more Wilder fights are necessary.
So here's one take.
I agree with @mikeod that Fury vs Usyk would be a dreadfully awkward and not fan-friendly fight. I've seen enough of Fury in the past that this is certainly a distinct possibility. Should Joshua get a Usyk rematch next? Probably not. The consensus is he'd lose just as badly. A second loss to Usyk would certainly jeopardize Joshua's future as a boxing draw.
I don't claim to have the best answer... but somewhere in there I think the public would love to have a Wilder-Joshua fight. Yeah, they've both been beaten, taking the luster the fight would've had if it had taken place before the 1st Fury-Wilder fight. But why should either of them retire just yet? Wilder has only been beaten by one man. Joshua by two, but he avenged one of his losses.
The styles of Wilder and Joshua would make for a great fight. Sure, they're both damaged goods. But one could surmise that from that fight, the victor could look toward greater things, while the loser would surely be one step closer to retirement. Usyk of course, has earned his right to dance with the big boys. But his lack of size in this current age of behemoths doesn't bode well for him. Call me crazy (please)... but a well-focused, "head-on-straight" Wilder would very likely take out Usyk within the distance. People like to rag on Wilder's lack of boxing skill, because let's face it... first impressions are hard to erase. But Wilder HAS taken some steps in the right direction since his Atomic Windmill days. He IS trainable, IMHO.
Fury will probably retire at some point before the current crop of big, dangerous heavyweights get around to where they can challenge for the title. So what does that do for his legacy?
I'm sorry to say this... but other than his great trilogy with the dangerous Wilder (who nobody wanted to face)... his resume becomes pretty thin rather quickly. Wladimir? Please. That wasn't so much a boxing match as it was a psychological contest, which Fury won hands down. The worst heavyweight title fight in history.
But let's say Fury defeats Usyk... then Joshua fights Wilder and comes out victorious. Ahhhh.... THEN, Fury could face Joshua and have the chance to defeat another championship caliber fighter convincingly. That would certainly help Fury's legacy. Odds are it'll never happen, though.