The Rock is very much in play for Wrestlemania 31 next year, however, he's not yet confirmed. Rock stated over the summer he has a verbal agreement with WWE for more match.
There were also plans at one point for Rock to oppose Brock Lesnar at Wrestlemania 30 this year but the plan was nixed (according to Rock) due to injury. Lesnar is still someone the Rock wants to face, feeling it's the biggest match possible. Vince McMahon has been enthusiastic about such a match, while Triple H has pushed to do Lesnar vs. Roman Reigns.
Whether or not The Rock returns to WWE will hinge more on opponent and match at Wrestlemania more than any other issue. Obviously pay is a factor but The Rock doesn't want to come back to take up an undercard role. He wants to work in the main event, a main event bigger than this two matches against John Cena.