As of right now, John Cena doesn't have a match at WrestleMania. The reason for that is because WWE is still unsure what's happening with Lars Sullivan after apparent anxiety issues led to him taking a seemingly indefinite leave of absence from the company. The original plan was for Sullivan to attack the leader of the Cenation to take him off the Royal Rumble card and set up a bout between the two.
In reality, Cena is shooting a movie and it's said that management is currently understanding of Sullivan's situation and are happy to have him back to proceed with his main roster debut when he's ready.
Whether or not that will now include a big push and WrestleMania match is hard to say but WWE may be reluctant to move ahead with that if they can't rely on him showing up when he's needed. As a result, don't be shocked if they see how he gets on in the midcard for at least a few months.