Sheamus is not a mega-heel. He's an upper midcard heel who will become a main eventer soon and how long it takes for that to happen depends on how long this title reign goes. His name is nowhere near Jericho or Orton yet, but he may get there one day.
He's a main eventer right now. The fact that he holds the title puts him up in the main event as we speak. I mean, he's feuding and beating the crap out of Cena on a weekly basis. They've set him up as a mega-heel; the crowds are just a bit slow on the uptake, but he gets more and more heat every week. I mean, you can't determine the status of a wrestler by his skill in the ring. That just determines how good of a performer he is.
Like, he might be a crappy main eventer as we speak, but he's still a main eventer with the WWE title.
Cena can feud with Legacy, DX (again....), and even Kofi as a face. He's had face VS face feuds before, they have a match out of respect to see who the better man is. Not as entertaining in the long run if they do those one after another, as he needs a heel to bring out his rage.... but it could still be done. Cena could face anybody and still make the feud great.... and that includes face VS face feuds.
As much as I like face vs. face respect feuds, they get old quick, and they get boring with Cena because he does them all the damn time... or at least he tries to. He has this air about him where he feels as if he's competing against someone in a respectful battle of wrestling skill, rather than fighting against this giant, evil Irish guy who just wants to see Cena unconscious while he holds the belt.
Jericho challenged Orton for the title in December 2007. He turned heel on HBK in May 2008. Nice try.

He was great as a face though, agreed.
lol Give me a break, it still was a few weeks, with a mini-JBL feud in between where he set up his character to be rather violently vengeful before he went into his feud with HBK as he transitioned to a heel. That feud was necessary for his character, so that feud lasted like 3 weeks.
The point being, he was in the main event with Orton, that feud kept the crowd's eye on him throughout his short feud with JBL, and that feud with HBK brought him up even more steps. If the HBK/Jericho feud wasn't a main event feud, I don't know what was.
Exactly. That's why they need to wait to turn Cena. They should wait until they REALLY need a shocking moment. His turn will be up there with Hogan's and Stone Cold's because he would essentially betray his fans (other than those like me who have remained fans his whole career).... WWE should not waste such a massive opportunity because once Cena turns heel, that's it. They will have done the biggest turn they possibly could make in a LONG time.
They have about one million and four opportunities between this monday and forever from now to do something shocking with Cena. He could do something with Bret on monday, seeing as Bret is set to come back for the next few weeks. I can't really think of anything else off the top of my head, but believe me, the WWE has a few situations already set up where they could add a Cena heel turn. Or they could easily set something up to draw a bunch of fans for the TNA war.
It's not that I see a Cena heel turn in a negative light, what I see in a negative light is WWE wasting it when it could be HUGE if they wait long enough until they really need it. WWE does not need that big of a swerve right now. WCW did back then and it was huge. History can repeat itself. Cena's staying a face until another huge groundbreaking swerve is needed. Trust me on this one.
Well when do you think they should write something big? I would imagine if they were planning something, it would be after Bret Hart. If they threw everything at us at once, things would get convoluted, so they may have a bunch of shit planned out to compete with TNA in the next few weeks/months.
If Cena WERE hinting at a heel turn, he's taking his time.