According to PWInsider, the idea to have a champion vs. champion match at WrestleMania 30 next year has come up within WWE. The idea is to have the WWE Champion take on the World Heavyweight Champion; whether this would be a Undisputed Title unification, or just a match for both remains to be seen.
There is a current push to bring the World Heavyweight Championship back to its former glory, which is why 14-time champ John Cena is currently holding the title. If both current champions retained until April, we'd be looking at John Cena vs. Randy Orton, which could be an interesting play on Orton being called the "face of the WWE" as of late. The two have never faced off at WrestleMania in singles action; they were involved in a triple threat match along with Triple H at WrestleMania 24.
Earlier this year, CM Punk also voiced his desire to face John Cena, who he considers to be the ultimate opponent for a championship match at WrestleMania. It's possible Punk could transition back into the WWE Championship picture before April. He is currently involved with Daniel Bryan in a budding feud with the Wyatt Family.