Regarding Hulk Hogan, it'll have to be one of the most structured and devious turns in professional wrestling for Cena to top Hogan's. Hogan, like him or not, was the forefather of what we see today. Had it not been for Vince McMahon creating "Hulkamania", we'd probably be watching fifteen different wrestling promotions a week on local cable channels. Hogan was the "boom" behind putting wrestling on the radar. He connected to children and adults alike, did commercials and movies, probably did some volunteer work to help his image, and played the "American hero" role that no one could humanly dislike. When Eric Bischoff took the risk of turning him heel, it took "Hulkamania" off the map and enforced a new side of the "American hero". That, beyond all else, was easily one of the greatest heel turns of all time.
Stone Cold's heel turn was a different cup of tea. He didn't play the role of the "American hero", nor did he do anything else that came with it. He was actually a polar opposite of Hogan but people around that time were much more immune to the new material (due to ECW and the Attitude Era implanting the idea of blood and cursing being a requirement in professional wrestling). The general audience at the time liked that shtick. Stone Cold turned heel not by changing his image, but by siding with the most hated man in the company, Vince McMahon. That's what honestly adjusted Stone Cold because in Hogan's era, he would've already been known as a heel.
Twelve years later, the audience has been brainwashed into thinking those "favors" of blood, sex and cursing have been stolen because of WWE's recent PG-limited program. They refuse to watch anything outside of WWE (except for TNA, which according to them isn't good enough either). John Cena's family-friendly personality just doesn't fit their standards of what they "saw" in 1997-1999. It's too much wrestling for them.
If John Cena were to turn heel, it'd be much more controversial than Stone Cold's but if he planned on topping Hulk Hogan, it would take a lot more hard work and a better angle.
BQ: Probably. Trust me, when Vince McMahon steps down, he'll have a laundry list of things that he wants done immediately and in the future. He'll have an impact on wrestling even in retirement and eventual death. But, if that's not the case, than Triple H will have to input everything he knows from his prime in wrestling to surpass his father in law. He actually wrestled, unlike Vince McMahon (professionally), so that might be a helping factor. The tag team division has grown and the diva's division is supposedly said to expand in 2013, so that's a light at the end of the tunnel.