#2 Samoa Joe

If WWE wants someone established to face the Rock then Samoa Joe would certainly be a perfect choice for an opponent in his eventual return match.
Samoa Joe is a natural heel and his combat psychotic attitude would work greatly against the Rocks over the top personality.
Even that Joe never had a Wrestlemania match this would also be a good way for WWE to reward him for all of his hard work since joining the company giving him his debut on the grandest stage of them all against the Rock.
#1 Brock Lesnar

In terms of dream matches and star power the absolute biggest match at its disposal will be putting the Rock against Brock Lesnar, something that was actually meant to happen several years ago.
Due to injuries that The Rock suffered against John Cena, the match never took place but there have already been rumblings that this is what Vince McMahon wants to happen Wrestlemania 35.
Did a WWE Hall of Famer predict John Cena's heelish ways long ago? More details HERE.