As much as everybody in the world wants to see Rock/Cena at WM it just sounds too good to be true. I doubt it will happen, the more likely outcome will be a stare down or something after Cena's match with Miz then they go at it for a bit with Rock coming out on top, then afterward he raises Cena's arm and celebrates with him.
However, if by some huge stretch of unprecedented creativity that Rock/Cena officially comes to fruition at WM, you know that they would put Orton up against Miz before they would consider Morrison. They may even add Punk in there and have a triple threat. Both those options would be a million times more likely than Morrison getting a shot. As much as a Morrison/Miz match would straight up be 10 times better than Orton/Miz, and actually have legitimate backstory, Morrison wouldn't be able to sell a WM feud for the WWE Championship at this point in his career, and this is coming from a big fan. He's getting closer as the weeks go by, but not right now on the road to WM. He wasn't even able to get in a PPV championship match at RR, no way they jump a million miles forward and propel him to the mainevent of WM. As far as mainevent title shots for Morrison goes, my bet is on Summerslam later this year.
Now don't get me wrong, I would fucking love for Morrison to win the Chamber, but you gotta believe that business wise its not the best option. If somehow Morrison did get into the title match it wouldn't be 1 on 1, it would need to be a triple threat or fatal four way where they can count on someone like Orton, or another big face who will guarantee draws, as opposed to the risk of throwing a guy like Morrison, who's never even mainevented a B PPV head first into the deep end that is Wrestlemania.