Del Rio's not ready yet. His character is pretty awesome, but he's nowhere near good enough in the ring, and despite the disdain for the word "wrestling" shown by representatives of World (redacted) Entertainment, he's still pretty raw. Plus we've seen what happened to the last two guys (Sheamus, Swagger) that got world titles early in their careers. Let him lose a competitive match to Edge, have him deal with someone like, say, Kofi Kingston in a secondary feud for a couple of months, then prime him for another run near the top later in the year or next year. He should be around a while; no need to rush it.
They've painted themselves in a corner with Miz/Rock/Cena. If Rock costs Miz the title, it'll be both predictable and fly in the face of all logic (thanks to Vince Russo, the two are no longer mutually exclusive concepts). If Rock costs Cena the title, you end up with a heel Rock that nobody wants to boo against a face Cena that only some people want to cheer.
If you have Rock cost Cena the title but have Cena turn heel in the process, it's going to require some really ornate machinations to pull it off, and wrestling doesn't generally lend itself to complicated character changes. Plus it leaves you with heel Cena vs. who exactly? If Rock is around for a while and is committed to wrestling just enough to maintain a feud until Summerslam, then fine. But if Rock fades back into Hollywood, you're looking at Cena, Miz, Sheamus and Punk on the heel side and unlikable Orton, HHH and niche Bryan on the other, with Sin Cara an eventual possibility (unless he goes to Smackdown to play with Mysterio and Del Rio) and Jericho a wild card. Even if Rock does hang around long enough to get a decent feud with Cena, they have to develop a top face really quickly to fill the void. And none of the top heels are in position to turn face in the near or mid-term future, unless they do something wonky to hotshot it (I've seen proposals for a Cena/Miz double turn, which would be awful, because Miz is not a face and double turns almost never work -- even Austin/Hart at WM wasn't a double turn itself, it was merely a defining moment in their characters' evolutions.
Want a major turn? Have HBK help HHH cheat his way to breaking Undertaker's streak. HBK then manages HHH as heels. Triple H is best as a self-absorbed bad guy anyway. Then maybe a Summerslam match between HHH (with HBK in his corner) vs. Taker (with Tom Brady in his corner as an 18-1 empathizer). Or maybe not.
Or you could go really wonky (and knowing that old vs. new feuds never work, no matter how appealing they sound) and have Austin, Rock, HHH, Undertaker, Trish and perhaps Booker T and Kevin Nash bemoan the state of WWE, how they all retired or took on reduced workloads to usher in the next generation of superstars and divas, only to have them be huge disappointments. This would set the stage for a Survivor Series match. Problem is, who would you be more likely to cheer for: Austin/Rock/Undertaker/HHH or Cena/Orton/Miz/Del Rio? Because you need the old-timers to be the bad guys in that confrontation and the young guys to ultimately win, since they're the ones that will be around consistently going forward.
