I think it's most likely going to be someone from a former Soviet-bloc country, the leading 'next great heavyweights' in my book are,
Wladimir Klitschko of Ukraine
He easily outboxed and outpunched Chris Byrd and earned the IBF title.
Sergei Liakhovich of Belarus
He had put on a great show with Lamon Brewster and now holds the WBO title.
Oleg Maskaev of Kazakhstan
If his rematch with Rahman goes well, he'll hold the WBC title.
All three of these guys are good boxers with a lot of potential. Maskaev's chin may be a little suspect, but so was Klitschko's .. undeserving, if you ask me. Anyone of these guys can throw enough power to knock anybody down.
Of course, I'm a fan of Baby Joe Mesi. He's still undefeated. But his last couple of wins were struggles, especially with Jirov who had just stepped up from cruiserweight, if memory serves. Joe hasn't been fully tested against a true heavyweight contender, and at this point, since he has suffered multiple subdural hematomas on his brain, I just cannot support his continued fighting. I liked to watch the guy box, but now it's just a matter of time before he gets seriously hurt.
Of all of these guys, I like Klitschko's chances the best. With Manny Steward in his corner he seems to have settled into a better strategy for winning fights .. accentuating his strengths, staying away from the opponent's strengths, and maximizing his opponent's weakness. He just systematically demolished Byrd from the start of the 2nd round. Unless he falls back into bad habits of punching himself out or moving straight back and suffers another huge knockout loss, I think he'll be on the move toward the next great heavyweight.
Of course that's just my opinion, I could be wrong.