At first glance, you could easily say Evan Bourne. He's young, he's incredibly exciting when it comes to the high-flying aspects of pro wrestling, and he seems to be (at least from what I know) someone who doesn't particularly cause problems or bitch about his spot, so he might be around for quite a while. However, a huge part of Mysterio's popularity comes with his mask as kids (and adults alike) love their superheroes and that costume gives him the appearance of one...and that's something Evan Bourne is severely lacking.
Sure, he's fun to watch, but he's got no persona, nothing that stands him out from the rest outside of the flips and such. If Bourne is ever going to become as popular as Mysterio, he's going to need to develop some sort of identity that fans that click with. Now, I'm not suggesting they randomly slap a mask on Bourne and just be done with it, but something needs to amp up his appeal. The same would go for all the other potential candidates, though most of them I feel have qualities that better suit them for NOT being an ersatz-Rey which will prevent it from happening in the first place. Take Kofi Kingston. In my opinion, he's always shared much more with Booker T than Mysterio. I think we're maybe a year away from seeing Kofi turn heel and become a bit rougher around the edges. Justin Gabriel, too short of a career to tell where he's going, but he's in a similar boat to Evan Bourne though a bit more grounded. We've got the likes of "Hunico" in FCW, so there's some potential for a revisit to the luchador concept, though again, much too early to really see where that goes.
All in all, I don't think we'll really see a definitive "this generation's Rey Mysterio". Everyone tends to put their own spin on things and if they get popular enough to be thought of as on-par with someone like that, they tend to have their own definition. Chris Jericho started off as potentially "the new HBK", but instead, has created the likes of people (including myself) saying "the Miz is the next Chris Jericho" instead.