Hopefully it's Lesnar. Cena will be too preoccupied with the Authority (against either Orton or Triple H) or too busy putting Bryan or Punk over in some capacity with the WWE Championship on the line (not in a one on one match, but something like Cena/Orton/Punk or Cena/Orton/Bryan or Cena/Punk/Bryan) to really be able to build up an epic match with Taker for this year. Cena/Taker can wait till 2015.
Brock is the way to go for next year. I wanted to see this match badly this year and I don't want to wait another year as it could mean we may never see it. Brock and Taker have history to build off of, Undertaker and Heyman have recent bad blood stemming from this year's Mania and Lesnar can easily be sold as a huge kayfabe threat to the streak. (He's won his last two matches and will win his third if he wrestles at the Rumble as rumored... that's the longest collection of wins in a row since he's been back, so he'd be heading into Mania with a lot of steam.) Even more than with his last two matches with Triple H, I want it to look like Taker has just been through a war when he survives Lesnar.