They don't really have to make Cena a heel, though, there's no way he WON'T be boo in Rock's hometown of Miami. Plus, he's already boo mercilessly because as Kev said, people do not like him! The older fans and those who grew up during the Attitude Era (like me) will continuously boo him, to me he represents all that's wrong with the WWE today.
I would like for WWE to make him a heel just because it would be refreshing.