My guess is that at Wrestlemania 27, it will be Undertaker vs Kane (for the 3rd time).
Kane will destroy Undertaker at Hell in a cell and set him on fire, so Undertaker will be out for like 4-5 months and then he will come back a couple of weeks before Wrestlemania 27 fully healed, and challenge Kane and win for his 19-0.

And maybe still too early, but in Wrestlemania 28, I think they will have Undertaker collide with Cena while he's looking for a whorty opponent for his 20-0.
He will be searching in Smackdown and a lot of wrestlers will ask him to be his opponent at Wrestltemania, and he will say no to everyone. Then he will start to look in the Raw roster, and same thing happens, but then, he will pass in the corridor in the backstage and crosses Cena, but Cena will ignore him and continue to walk and won't ask him for a challenge, and will later say something like he can beat the Undertaker, but doesn't want to break his streak. Then Undertaker will feel insulted and will start a feud with Cena and want him at Wrestlemania 28. Then he will win and retire.