I would argue that gives Cena all the more momentum as a top villian if he were to turn at Wrestlemania. A top heel that can't be stopped is something WWE has failed to create in years...about a decade actually.
And Austin won the Rumble in '01...just an FYI...
I don't think the turn in this case is about a superstar being turned on, but rather the "Cenation" being turned in. One could argue that Austin turned on the people, but I would beg to differ as I think it was a completely different situation. Austin, while a hero, was never a fighter for, or defender of the people. His character was a lone wolf, anti- hero, that was always in it for himself. The fans were more or less just along for the ride, where as with a babyface like Rock, Hogan or Cena, the fans are what fuel them. The People/Hulkamaniacs/Cenation are just as important a part of those characters as the ideas they fought for. But Austin? He was different and his turn on Rock was truely done for his gain, not to prove anything to the fans...because for the Austin character, it never was about the fans to begin with.
I think the OP makes some good points, but I think too much is being put on "what Austin did". While there are similarities, the turn of John Cena (whenever it does happen) will be different from those of Austin and Hogan because the motives are so dradtically different. If Austin had faced HHH at Wrestlemania 17 and went to desperate measures to beat him, then a parrallel could be drawn between Cena and Rock - but that's not what happened. Austin faced Rock, who he had already beat at Wrestlemania for the title, two years prior so he wasn't trying to avenge anything. He wanted the WWF title back and was going to do whatever he had to to do so. Cena needs this win, not for the title, but to finish what he failed to finish the year before in the "biggest match of his career". Cena is not fighting for the fans going into Wrestlemania 29, or even the boys in the back like many would say he was doing last year, but rather he's fighting for himself. THAT is the biggest change I'm seeing in Cena right now, and think is the greatest possible indicator for a heel turn to ensure he gets the job done this year.
As many others on the thread have stated, I'll believe it when I see it. However, I do think the possibility for it is stronger than ever as I think WWE realizes two things. #1: There are/will be very few ways and opportunities in which Cena can be turned at this point...the stage has to be the biggest (only Mania will suffice) and the opponent has to be larger than he is (which at this point is only Rock). #2: Rock vs Cena II is far too predictable. Something needs to go down in this match to add the flare to it that it currently doesn't have. WWE needs something huge to carry the company post Mania once the likes of Rock/Taker/HHH go away, and a fresh John Cena as the top villian in the company (as WWE Champion mind you) could be just the answer they are looking for.
That would be a good heel promo, "last year I did it for you guys, and it got me nowhere, because I cared for what you thought, just before I mocked the rock's peoples elbow at mania 28(remember the look on his face before he mocked rocks elbow, as if he was going to turn heel), I was contemplating on cheating, but then i thought of all of you, so i changed my mind and committed the greatest error in my life, for what? for you, I lost my wife because of you, my confidence, my hustle, my loyalty to myself and my respect, I had the worst year of my life, and when i finally think forget the "cenation" and fight for myself and do what it takes(cheat, heel turn at mania) I finally beat the rock, so I'm done with y'all, because ya'll got me nowhere, my time is now." something like that.