I know this has been done to hell and back and it is very unlikely to happen.
But Mania is the perfect opportunity to turn Cena heel.
WWE are expecting him to get booed out of the building when he wins, so why would they have that in the background of a career milestone, imagine a Cena career highlight DVD years down the line, they will for certain have Him VS rocky in there.
So why not make the crowd reaction fit.
It could be simply done, have him hire the Shield, to help him take out Rocky, then complain about Rocky been given the title despite only working less than ten shows, while Cena has worked his ass of for years, every day of the week.
The you could have him on a dominant run, while building up faces to challenge him, imagine Super Cena also with the mindset of 'I'll do whatever it takes to win... even if it means bending the rules', have him keep the title until next Mania, and then have the next big thing take it from him.
I know my wording is awful, and this is highly unlikely to happen, but I just felt like throwing it out there.