For the life of me, I don't understand why so many want it to be Kane. Kane isn't the monster he used to be and I really don't need to see a third match between them at Wrestlemania (their 837th match overall) when there's obviously better alternatives. A match with Kane at this point is not a big enough match for Taker to close his legacy out with (his retirement match would obviously be the last match of whatever WM he retires at.)
For me, his final opponent entirely depends on whether the streak stays intact or not. I think it certainly should at this point, and in that case, the storybook ending to Taker's streak and career is John Cena. Undertaker comes out next year and says that win, lose or draw, the streak finally sees it's end at Wrestlemania 30. He gives a promo about how for the past 23 years, he has slayed all the dragons that have come his way (this is Taker speak that basically means all the people he's defeated throughout the years) and that the time has finally come to lay to rest his final enemy. He says that when he looks around, he sees the biggest challenge left in the locker room is John Cena (he doesn't come right out and say this, there is some anticipation built up to it.) Cena's music hits, he has a curious but confused (perhaps intimidated) look on his face as he walks down the ramp and tries to get Taker to better explain the reason for the challenge he's just laid down.
Taker explains that ever since Cena rose to prominence, he's become the most dominant man he's never beat at Wrestlemania or ever for that matter. He also explains anytime Cena has come up against many of the same people that have fallen to Taker at Wrestlemania - HHH, HBK, Punk, Henry, Orton, Batista, Edge, his own brother Kane, Big Show - Cena has come out on top. (If you wanna add non-Wrestlemania names that Taker has defeated as well, you can add Khali and The Rock.) He says that fate has kept them apart this long for a reason but come Wrestlemania XXX, they will finally meet and one way or the other, John Cena will be the last man the Undertaker goes up against (win, lose, or draw!)
I can't imagine the heat for the match leading up to it, with many people thinking there's a chance Cena could break it. Especially with the super smark crowd that will hate Cena's guts already. I also wonder if the WWE Title should be incorporated into the feud (probably not, but I do believe Cena should be kept mostly dominant for this whole year so he can go into this feud and match looking the strongest he can look.)