Two guys who could theoretically do it (although I don't think they will get close, but neither will anyone else) are Ballan and Leukemans.
Ballan already won Vlaanderen, could definitely win Roubaix. Already finished top-10 in Milano-Sanremo twice, with luck he could certainly win the same way Cancellara did. Of course, LBL and Lombardia are a different story. He did finish top-20 in LBL once, and his world championship win was on the difficult Varese parcours, all the stars would have to allign for him to win those races, but of the guys who can win Roubaix, he is certainly one of the better climbers. The biggest hurdle is of course he has to recapture his 2006-2008 shape, that seems highly unlikely.
Leukemans will probably never win a single monument, but he is one of the most versatile riders out there. He was pretty much a climber type, for races with small hills, untill he tried Roubaix once, and was suprisingly good. He at least has the capabilities to finish top-10 in every monument.