Raoul Duke
Gordon Durie
Haye will definitely challenge again if he stays injury free but that's a big if. Nobody has the speed and power combination that he has in the division. I'd like to see him beat one of the American pretenders (Jennings, Martin) next before fighting someone like Ortiz who is making waves but is also untested. Then he should get a title shot.
America is desperate for another heavyweight superstar which is why Wilder has had such an easy ride but I think he loses to Fury, Klitschko and Haye comfortably.
As for Joshua, he will need to learn to move his head if he wants to progress but it's still early days for him.
I wouldn't call Jennings a pretender. He's never had a big promoter or easy path. In his last five fights he's faced Fedosov, Szpilka, Perez, Klitschko and Ortiz. The five before that had a combined record of 110-16. He only has 21 fights, 15 less than Wilder. He lacks size and power, though, so could be a decent step up for Haye or Joshua.
Ortiz, though untested, is the real deal in my opinion. He's got everything.