...but unifies against Wilder for the vacant, and defends against his WBO mandatory (Parker or Joyce, your choice), WBA mandatory (say Dubois or Helenius, your choice), and has a good swan song against Ruiz or someone comparable, where do you rank him in history, as a heavy, and Pound for pound? So he wins five more, unifies all the heavyweight titles to go with his cruiserweight collection, but lacks the big, gaudy name on his resume. I wan't include a poll. This is a free-form discussion. I realize that most people believe that Fury will be back, and he probably will, but please, let's just go with the premise.
It's difficult to rank someone at Heavyweight when they did not spend even the majority of their career there, let alone all of it. That being said if he unifies the four alphabet titles at heavyweight immediately after doing so at cruiserweight he would easily have to enter the pound for pound discussion. Where he gets placed is determined by the eye test going forward and who he actually faces. You will always have those who will detract from Usyks greatness if he can't fight and beat Fury.
Fury has to beat a bodybuilder or an aging middleweight to reclaim his belts because they are still his belts. He's clearly preparing for it & expects Usyk to win. He's staying in shape because he knows he needs all cylinders for Usyk or he'll lose an ugly decision. If AJ can execute everything he has even if it means getting folded in the process people will realize that yes Fury is step too far for Usyk....but no matter what it always was and just to be competing with heavys is a great achievement.
I suspect it would be difficult to place too high without the Fury win. Likely in the low end of the top 20, above say a Riddick Bowe and a couple above Fury himself. He could push himself higher by extending his career even if he picks up the odd loss, but time wouldn't be on his side. But definitely an ATG if he becomes undisputed, as he starts a new lineage with or without Fury. Plus you have fighters like Gene Tunney who similarly didn't campaign long at heavyweight, but are considered ATG in the weight class.
Wilder wouldn't fight Usyk so that scenario is totally unrealistic. Wilder will fight Helenius or Ajagba types as usual. I still believe before Usyk retires he easily proves himself to become an ATG. Greatest CW of all time already and I believe he will cemwnt himself as the best HW post Lewis (already is the best at present).