As I said over a month or so ago, it'll likely be Rock/Brock/Cena.
Wrestlemania is about bringing in non-wrestling fans and Rock-Brock would do that because one is a mainstream Hollywood star and the other is UFC's biggest draw ever. If you throw in Cena, you have WWE's own top star right in the middle of it. I'd imagine that'd get a fair bit of publicity. It's probably the biggest WM main event WWE can produce this year. And if you have Undertaker and CM Punk in a Respect vs Streak match, you have a hell of a double main event.
My only issue with it is Brock competing for the belt, as I don't know if it fits Brock's character and how they would fit that in there (remember that Brock is fired in kayfabe terms and also doesn't want back anyway) but they can figure something out. Just seeing their three faces on the WM29 poster made me think this could be the headliner.