Well, as an old school guy, I REALLY liked the idea that was mentioned above, of Taker actually winning the Rumble, and then turning the Rock vs Cena confrontation into a three way...
For one, it would be the BIGGEST match Vince could possibly put together at this point in time. Taker will leave soon, it will probably be his last WM appearance to go for the 20-0, and what a way to go out it would be for the Deadman to face both The Rock and John Cena in his last match, and still come out on top somehow. Would definitely be a GREAT way to send the Deadman riding off into the sunset - and personally, I wouldn't want to see a Triple H/Taker rematch... mostly because they did the rematch thing already with HBK and Taker... and I honestly also don't want HHH to go over Taker at Mania, which I somehow feel WWE would be going for, since they probably don't want to repeat the HBK/Taker story with the two matches, and HBK losing both.
Secondly, I think a triple threat would be good on Taker, since with his increasing troubles physically, in a situation like that, he wouldn't have to carry the match all by himself all the time. He could work a bit at the start, but then somehow get taken out of the equation for a time, while Rock and Cena have their moment and it looks like the match will be decided by those two... and near the end of the match, Taker comes back in and manages to score the win.
And of course, with the title being on the line as well (given Cena goes into WM with the belt, which I think he will, and Taker winning the Rumble), it would be an even bigger match. And it would add to the drama... people would think Cena will win, because neither Taker or Rock could/would do much with a title... but at the same time it would be likely that Taker would win, because of the Streak. Only Rock seems like a bit of a "prop" here since he is not really in question of winning the WWE title one more time... So in a Rock vs Cena only situation, with Cena as champ, it's really obvious who will walk out victorious; same with Taker vs a single opponent, even if it's Triple H again. But if you mix up both aspects, streak vs title, with Rock being the X-factor... That could make Taker's (potentially) last match really great and interesting, and also add to the drama between Rock and Cena.
Also, I think it would be no problem giving the belt to Taker at his last WM, and him basically leaving as champion afterwards... Hell, if Vince did it for Edge (who, as great as he was, is nowhere near the legend status that Undertaker has), he sure as hell could do it for the Deadman. Taker goes over both Rock and Cena at WM, wins the title one last time... and then announces the next night on RAW that there is nothing left to do for him anymore, and that his time has finally come. He forfeits the title, leaves it in the ring... and ends one of the greatest careers in the history of this business. Yeah I know, I'm a Taker mark, but that's really the way I'd LOVE to see it go down, haha.
Hell, you could even add Triple H to the mix by having him win the Elimination Chamber for a number 1 contender's spot, effectively turning the match into a fatal four way... he'd have his rematch against Taker, at the same time a chance at the WWE title, plus Rock and Cena in the same match... unfortunately, a match like that is very untypical for a Wrestlemania main event I'd say, and maybe also not quite suited for it...
But realistically, I suppose it will be Cena vs Rock in singles competition (since after all, really only One-on-One matches have that "epic" showdown feel to them), most likely for the title though... and Taker will face someone else, since his WM match is a marquee match in and of itself, no matter who he faces, because of the streak... and thirdly we'll see the WHC taking on the Rumble winner... and of course it's quite smart to have three big matches [one huge (Cena vs Rock), one almost as huge (Taker/Streak) and one big (WHC because of the title on the line)] instead of "wasting" Taker by scrapping his singles match and putting him in a mix with Cena/Rock, but I'd still prefer to see that as it would enable Rock and Cena to battle at Mania without losing face (if both lose to the Undertaker, there's never shame in that), and also enable Taker to go out on the highest note possible, as champ after defeating the biggest stars of this era as well as the (late) Attitude era (after Rock had achieved quasi-SCSA levels of popularity) in his final match... and if necessary, Rock and Cena could have a final showdown at a following PPV (maybe Summerslam, even) and finish their feud in a one on one match then.
But yeah, as said... realistically, I'm rather expecting three singles matches for the WWE and WHC titles as well as for Taker.