OK as I say, I don't think that Stipe vs Ngannou II is going to happen any time soon : Summer 2020 (if even then).
So....given that..... I would say 6 current UFC HW fighters who COULD be suitable as Ngannou opponents are :
Cormier
Pavlovich
Gane
Hardy
Volkov
Lewis
Now, 3 of those (Pavlovich, Gane, Hardy) are quite new to the UFC and definitely rising up quite fast. But all 3 look promising, in their own way.
Cormier is v technical, he comes with a solid game-plan to win each fight, but....he has said he is only gonna fight once more and that will be Miocic.
Volkov is a good opponent and ....Ngannou declined (mitigating circumstances : short notice)
Lewis has already beaten Ngannou in a REAL dud of a fight.
Given all of that, I would suggest that the UFC offer this fight next to Mr Ngannou :
- Ngannou vs Lewis II.
Why? Lewis is a big strong resilient guy, he can take a shot, and give a shot, and it will give BOTH fighters an opportunity to fix-the-abortion that was the first fight, and put on an entertaining fight. Ngannou's motivation would be to avenge that defeat, and make things right with UFC for such a non-event of a fight. Lewis's motivation is he beats the No.2 ranked UFC fighter Ngannou.
Lewis is fighting Ivanov this weekend in New York, so.... my bet is that if Lewis wins, then UFC will make Lewis vs Ngannou II, with a firm warning to both "Make this Entertaining, or else".