There are no guarantees in the WWE, but all signs point to The Rock dropping the WWE Championship to John Cena at WrestleMania XXIX. If that comes to fruition, then their rubber match must ultimately take place at Extreme Rules.
According to Wrestling Observer (via WrestlingNewsSource.com), The Rock is scheduled to appear at Extreme Rules on May 19 in St. Louis. While it is certainly possible that The Rock could be making a non-wrestling appearance, there are rumblings regarding CM Punk taking some time off after 'Mania, so it appears quite possible that The Rock will actually compete.
If he does wrestle, then his only logical opponent is John Cena. The Rock defeated Cena at WrestleMania XXVIII and will look to battle him once again at WrestleMania XXIX. This time the WWE Championship will be on the line and it is very likely that Cena will prevail. If Cena gets redemption and becomes the new WWE champion, then it will leave The Rock in a strange position since champions are generally given automatic rematches after they lose.
That may not apply to the The Rock since he is only around on a part-time basis, but it has already been said that The Rock will be involved with Extreme Rules in some way, so a rematch with Cena is only logical. If we are to believe that the WWE Championship is truly important to The Rock, then there is no way he will leave without taking a crack at getting it back.
Provided Cena wins at WrestleMania, the series between them will be tied at one. The next step could be a third match between them at WrestleMania XXX to decide who is better once and for all, but so many fans have been against the rematch at WrestleMania XXIX that I highly doubt the WWE can get away with The Rock and Cena main-eventing for a third consecutive year.
Saving the third match for SummerSlam is definitely a possibility, although that would make Cena's title reign far too predictable. Fans want to know that there is a chance somebody can lose, so if Cena vs. The Rock is on tap for SummerSlam, then Cena is essentially guaranteed to hold the WWE Championship for four months following WrestleMania.
The best way to satisfy The Rock's rematch clause without sending the fans into a fit of rage is to have the match at Extreme Rules. Even though Extreme Rules isn't technically a "Big Four" pay-per-view, the WWE is obviously looking to make it a major event if The Rock and potentially Brock Lesnar are going to be involved in some way.
Last year's Extreme Rules was great and it can be argued that it exceeded WrestleMania XXVIII. Many of the bouts on the card were rematches, but there were several exciting matches and Lesnar returned to compete against Cena as well. The thinking behind bringing Lesnar back at Extreme Rules 2012 was likely to keep fans interested in the aftermath of WrestleMania.
That could once again be true of Extreme Rules 2013, especially if Punk is going to take some time off as has been rumored. Punk certainly has his detractors, but he is easily the top heel in the company in terms of full-time wrestlers and he always delivers on the mic and in the ring. It is impossible to fully compensate for his loss, but The Rock facing Cena at Extreme Rules would definitely make up for it in terms of pay-per-view buys.
Once Cena ultimately beats The Rock at Extreme Rules and wins the three-match series, both men can move on once and for all. Cena will no longer be consumed with proving that he is better than The Rock, while The Rock can explore new and exciting feuds as well. I have enjoyed the rivalry between Cena and The Rock, but it doesn't need to last beyond Extreme Rules.
From both a financial standpoint and a storyline standpoint, The Rock vs. Cena at Extreme Rules makes a ton of sense, so hopefully the WWE plans on going that route once WrestleMania XXIX comes and goes.