It's a hard one to say. No-one would have picked Soderling in 2009, when just a few weeks earlier he lost to Nadal 6-1, 6-0 in Rome. If someone other than Djokovic is going to beat him, then I think the best chances are in the early rounds. Perhaps Dimitrov if he was playing perfect tennis? But that would be a really long shot. Gulbis perhaps - he beat Berdych in R1 of Wimbledon and loves a big occasion. He's taken a set off Nadal on clay before too. Unlikely though.
I actually think Del Potro probably has the best chance of the top 8 players (bar Djokovic). He played really well against Nadal in their Davis Cup final match. But what goes against him is the fact that he's often jaded by the time he gets to the latter end of slams (he led Federer by 2 sets to 0 at the French Open last year, but faded physically after that). But if Nadal is seeded 5, then they most likely won't play each other anyway, unless Del Potro upsets one of the top 4.
As for the rest of the top 8, past form at RG has shown that Federer can't do it. Andy's movement on clay isn't good enough (would love to be proved wrong there). Ferrer and Berdych don't have the belief against Nadal. And Tsonga's probably too inconsistent.
If someone is going to beat him, then they're going to have to be consistently excellent over 3-5 sets. Any dip in form and Nadal will take advantage and turn the match around. At the moment, I see only Djokovic as having that ability, plus the belief that he can actually do it (because he has now beaten Nadal at all the clay masters).
Good post, Katie. I agree with almost everything you said.
Andy's movement is okay but he somehow feels its compromised so he doesn't feel highly confident. Nole has proved since 2011 that he can beat Nadal but before then, he was just as useless so who's going to convert into what who knows. One thing I am sure of, Nadal isn't going to win RG forever and he has already maximized his limit.