Rock would not have to work a Chamber Match. In 2011, WWE Champion Miz wrestled Jerry Lawler and the Raw Chamber Match was a #1 Contender's match.
As for the CM Punk v. John Cena feud, if the writing is strong enough they can extend it to December. Other people can intercept at certain points especially at the Survivor Series PPV.
They could book it like this:
NOC: Punk outsmarts Cena and gets Cena disqualified or a quick roll-up
HIAC: In a HIAC match Punk barely escapes with a win, maybe from interference
SSeries: Team Cena wins
TLC: In a TLC match, Punk escapes with the championship
RRumble: Rock defeats Punk for his 10th WWE/World Championship
EC: Rock beats Punk in re-match; Cena wins #1 Contender's EC
WM29: Rock vs. Cena II - "Re-Match of the Century"
This is just one possible road they can take. During this entire period you should see Cena get more frustrated and question if he is the best like he claims to be. As the disappointing attempts to regain the title or a shot at the title (Royal Rumble Match) mount, Cena should start to become more determined to make it to WM29. He becomes more aggressive (edgier) wins the Elimination Chamber match then goes on to WrestleMania where he finally wins the gold again by defeating The Rock. After WrestleMania, Punk and Cena can have one final match at Extreme Rules (LMS, I Quit, or Iron Man) where Cena finally defeats Punk to retain the title and end one of the best feuds of this era.
Now this is not what I want to see but I'm trying to make a likely direction WWE will go in sound interesting. Cena's road to the title should be grueling and have some extensive build and storytelling to the triumph. It would be so underwhelming if Cena & Punk trade titles before 2013.
I really agree with everything you said. I can't say I would be mad if Rock wins the title, but I can honestly say I am really disappointed because he hasn't earned it. If all it takes is winning one match at wrestlemania and then leaving for 9 months to be the #1 contender, hell Floyd Mayweather should be pissed because he didn't get his title shot.
I think Rock should get challenged for his spot (Triple H? Lesnar?) and then go to Royal Rumble having somewhat earned it.
I was thinking, what if Cena is champ at RR, Rock wins, and disappears for 2 months. Triple H says he needs to make sure that the belt comes back to WWE to stay, and says he can only trust one man to do it, himself! Rock vs HHH one more time. If HHH wins, he would retire as champ, and there would be a tourney where in the finals: who else Cena and Punk renew their feud and have few more wars before moving on.
(I know that won't happen, but it would be insanity)