There's been several guys in the last few years who have had their name attached to the question 'the next MJ?'
Gay (for the deuce)
Probably someone else
I don't know if anyone is going to have the 200/400 capabilites that MJ had.
Wariner doesn't have the 200 speed, not yet at least, and probably will never run under 19.9, but he might get the 400 record.
If Merritt breaks 44 this season, he'll be the only runner, aside from MJ, to ever go sub 20/sub 44. If there's anyone who looks like they might be able to come close to duplicating MJs abilities, it is Merritt.