First off, it's not going to happen. And if it does, and I'm wrong, than AWESOME!
But, I'll suspend my disbelief here for just a second.
If the Great One is to make his return and wrestle one last match at Wrestlemania, it should NOT be in order to put over a younger talent. I know that sounds very WWE creative team of me, but it's the Rock!!! It's going to take a lot for WWE to get him to come back for a match. They're going to have to offer him a lot.
Besides that, there gets to be a point where a younger talent could actually bury himself by beating a legend. If the Rock, arguably the most popular superstar in WWE history, makes his return, and is somebody like the Miz beats him, the fans will shit all over it. Nobody will care about the Miz, but they will HATE the WWE for doing something so stupid. The Rock is THE people's champion, and his return should be against somebody huge.
The Rock vs. John Cena is the obvious choice, as they have a small history, and Cena is considered the Rock of the new generation. The two would have the most epic match in terms of pomp and circumstance, but Cena would have to play the clear heel. If they don't turn him heel, he'll become one. He won't get his normal boos, he'll get boo'd out of the arena worse than ECW in Philly!
The Rock vs. Orton is a match I think WWE should go with, if given the chance. They've danced before, but never at the big game. The two are great in-ring, but will the Rock give up enough of his time to train for the match? Will Orton and Rock get enough practice together. As good as Orton is, I don't think he could carry a ring-rusty Rock through the match.
The other big names in the WWE are HHH, HBK, Undertaker, Batista, Mysterio, Jericho, and Big Show. HHH will not want to put over the Rock, and the match really wouldn't make any sense. The Undertaker needs to have a great Wrestlemania, and a match against the Rock would be epic, but Taker can't lose, and the Rock has to win in his last match, unless it's a monster heel push for the opponent. Batista and Rock were both involved at Mania years ago... Mysterio vs Rock would be a good match, and the Rock has already said that if he returned he would love to work against him. Again, it wouldn't make a ton of sense, but it's a possibility. Big Show vs. Rock is not a match I want to see at all.
Jericho vs. Rock is a match that could go down, and is the only other choice I think the WWE has. Either that or HBK.
So,
Orton vs. Rock
Jericho vs. Rock
HBK vs. Rock
HBK would need to turn heel and be willing to lose at Mania, but it would be amazing. Jericho could use the big match, and they have a huge history. Jericho could talk about how big he's become since interupting the Rock during his debut a decade ago...
I would like to see Orton or Jericho go at the Rock, if the match is going to take place...