I understand some may say that Cena's act has been old for about seven years, and while I can't exactly argue that I'd say he'd really benefit from reinventing himself just a bit more, I'd still say there's still a few match types that still work with what he's going as now, like John Cena vs Brock Lesnar (which we're getting), John Cena vs The Undertaker, or John Cena vs Daniel Bryan. This character, as stretched out as it is, can still work.
The Miz should work on changing his character a bit more now, but I don't think it's really as needed as it's still not exactly completely run it's course. A feud with Chris Jericho or Christian would be great for The Miz. He just needs more ideas is the main thing. At the moment, he's not really having something to do, but that's because he's done almost all he can do with his character as a heel. Just, like I said, work something out soon.
I have to say beyond a shadow of a doubt, a man who just really needs to change is Orton. The Viper stuff was getting old in 2009. Come 2010, after a feud with Edge, there really just was nothing he could do to stay interesting. They actually had to have other people change gimmicks in order to give people something to want. His biggest problem is just that there was always possibilities with the legend killer because there were a billion legends. Now, Orton just seems like he lacks any real thing to do with him. His feud with Wade was turning out good, but Wade was injured before they seemed like they were going to finish it at Wrestlemania. They just need to end the Viper gimmick or stop keeping him in the main event scene.