Assuming they don't face each other in the rematch (no need in one really). I think Parker might be back at the Bivol - Beterbiev card against someone like Kabayel. Rematches against Whyte or Joyce may also be on the table. Zhang, I think, might face Bakole - Anderson winner on Usyk - Fury card in December. Or Ruiz vs Miller winner How do you see each of these possible matchups playing out?
If Usyk relinquishes another belt Parker should be first in line for a shot. Zhang probably the next most worthy to contest it too, ahead of the other contenders. But think I'd prefer to see Parker v Whyte II or Parker v Ruiz II.
Usyk should vacate the WBO belt, then Parker can fight for it. The only one that really matters for Usyk at this point is the linear heavyweight title.
I for one hope that Parker fights someone credible like maybe Kabayel. And then Usyk can fight the winner. I don´t want rematches, I want Usyk to face rest of the division: Parker, Zhang, Kabayel, Ruiz/Miller, Anderson/Bakole.