I could plausibly see it being Seth Rollins. At this point in time, nobody is jumping out at me as somebody that HAS to become champion. At Mania 13, there was no doubt that it was Taker's time. At Mania 14, it was Austin's. Mania 16 should have been The Rock's, Mania 18 HHH, Mania 21 Cena, Mania 30 Bryan... you get the point. Can anybody pick one guy who honestly deserves to be given that Wrestlemania moment right now? Perhaps if Reigns can build a lot more momentum in the next few months he'd be the top candidate, but he honestly doesn't seem that far from where he was last year in terms of readiness. I don't see Cena winning 16 unless Rollins loses the strap... there's no way they do Cena/Rollins again at Mania.
I guess it's possible that we see HHH and Seth face off for the title with H coming out on top and a Seth face turn... I highly doubt people would be okay with this match headlining though.
So yeah, I guess I'll go with Rollins for now. To think, if he holds the title until Mania, he'd have held the WWEWHC for a full year. Pretty impressive for a first run.