I think it will be Roman Reigns. He has the look, the impact finisher that gets the crowd's attention, he has that intense feeling and he isn't bad in the ring. He isn't the best worker in the entire WWE, but history goes to show that you don't need to be the best in the ring, and his current skill matches those, and even surpasses plenty that were world champions before. I don't think he's bad on the mic, but he still has lots of room to improve. Still, not to knock over Dean or Seth, I can see Dean being a WWE champion eventually, but not before Roman Reigns. I think Roman will be the tough face that is similar to Batista and Goldberg, where impact replaced their poor mic skills(except for Batista as a heel). I think that Roman could have a small stint as a heel too, he has that heelish look about him as well, but if I were WWE I would most definitely keep Roman as a face though, and I would mark if they put him over Cena clean(which I think he already got a pin on Cena clean in early 2013 if I remember right).
Big E Langston, is another that I agree could be the next WWE Champion. He has the same thing Roman has, the look and his moves leave an impact that gets the fans off of their feet, not to mention he already gets decent-above decent pops. The only thing that really needs to change with Big E is his finisher. I feel as though his finisher doesn't really have a huge impact like Roman's spear. Big E's power moves though, have a better impact in my opinion. That being said, Big E even has backstage respect with Cena, and others since he works out with them a lot. I say keep using him in the spot that's he's been at recently. Keeping him close to the big faces of WWE at the moment like Cena and Punk, and have him save them here and there while holding a feud with a heel just to keep him over with the crowd.
Cody Rhodes, is one person I could possibly see as a upset champion if he goes on to feud with Randy Orton, just for a short while. Orton has a huge number of singles wins against Rhodes, since their debut rivalry all the way to Rhodes's "retirement" match, Orton has always been on top of Cody Rhodes, and has always been the upper man. I know many will disagree, but I would like to see Cody give the Corporate a upset and win the WWE Championship from Orton. They have the history, and Cody is a very talented and loyal worker for the WWE, I think he deserves a small reign. I can wait though for him, but I suspect that if he doesn't take it from Orton, then it's going to be a couple of years until Cody gets the WWE Championship, if he even gets a shot at all.
That's all I can think of right now from the WWE currently.