If he wins he's fighting the Fury/Usyk winner, barring any rematch clauses affecting that. And even then he'd just hold out for them if he can.
If he loses he's obviously going to try for Wilder as at least one last pay day. Or someone else as he's still a top 10 boxer right now.
The dude is never returning to MMA unless he has to, and even then he's only going to do it so long as PFL hasn't cut his ass for refusing to fight for them. Then he'll show up to collect one last $1M+ pay and fuck off into true retirement. He'll never return if PFL cuts him and he has to try and sign somewhere else where he's not already been guaranteed his millionaire pay even if he'll be on a 3 fight losing streak and haven't competed in MMA in 3 years.
The guy's been done with MMA ever since he tore his knees for Gane and it's obvious to anybody paying attention. That's one of, if not the main reason why he shifted to boxing.