Aside from the occasional clinch, Fury doesn't rely on his weight. His whole style would benefit from slimming down as much as possible. Deontay could probably gain some since his style is all about the power.
I agree he could stand to lose more weight. I don't think it should be his primary focus, but I'm convinced if he was eating a clean diet and maintained his current regime he was drop a little more weight. It's more an issue of having some puppy fat. He's definitely not in the territory of someone like Chris Arreola or Greg Page, where the blubber is causing serious problems. It would be better to lose some fat, but he was obviously in good shape so its not a major issue.
There's jack **** Wilder can do. He already came in his best shape. Tyson will lose more weight, and will box even better now he's got the rounds under his belt.
Fury is what he is. I hate to drag out the old cliche again but boxing is not a bodybuilding contest (thank God). Unless Fury goes down the tubes again (which I don't think he will) I think he will look a little tighter in the rematch and be a few pounds lighter, but it won't be anything to dramatic. He just doesn't have the mesomorphic body type.
He obviously could be in better shape, he's been as low as 245lbs before, anything below 250lbs now would likely see him in his best shape. He'd be a bit faster, have better stamina and of course would be more elusive because of this.