You can't create a megastar. What you can do, is create stars, or, better yet, give individuals the opportunity to run with the ball. Some will be modest successes, some will fail and some will become big entities.
As for the subject within the subject as to whether or not Cena's a megastar or not... What Cena is, is a manufactured megastar by default. This isn't to say he wasn't going to be big anyway. He was. By late 2003/early 2004, it was obvious he was going to be a huge star in WWE and very likely the guy to take over the #1 spot (Batista leap-frogged over him with the insanely hot angle of rising up against Triple H, but in the end that spot did become Cena's). Cena was responsible for approximately 24-25% of WWE's merchandise revenue for the year 2004, when he was still considered something of a prominent midcarder. So he definitely had what it took to become a legitimate main event superstar with just the right amount of careful massaging from creative and Vince.
What WWE did, though, starting in earnest with the Triple H tapout to Cena at Wrestlemania XXII and the Shawn Michaels tapout to Cena at Wrestlemania XXIII, through the movies, albums, sponsorships, licensing deals, move to TV-PG (though the situation with Cena is only a relatively considerable, not earth-shakingly huge overlapping component with that move), all the way through the surprise Royal Rumble return, all the way through the final battles with Batista in 2010 and the subsequent Nexus storyline, all the way to "Once in a Lifetime," is say to the world in an unapologetic and enthusiastically resounding tone, "This is our biggest star, he's 'the face of WWE,' he's the man around whom this world revolves."
In the end, Cena's going to get his win back against The Rock. His superpush, his ownership of the domain, so to speak, is not yet over and it will only be through turning heel that we are given the major hint that his time at the very top is reaching its final phase. Even then, he'll always be one of their top guys, obviously, but one would think/hope that Vince and others see the enormous potential in Cena taking over the role Triple H inhabited from 2003-2006, as the top kingpin who serves as ultra-gatekeeper for the next wave of true top stars.
Sheamus will be given the role of Shawn Michaels in 1996, though Shawn's role back then was accidental. He'll be the "bridge" between Cena and ?????, the way HBK was between Bret Hart and Steve Austin.
Again, though, you can't wholly create new megastars. You can nurture talents into becoming marquee stars as Cena was given the chance to come along eight or nine years ago, and you can go from there. WWE's status as the wrestling promotion of America gives them carte blanche to take that hot star and declare that they are a megastar. In the end, if Cena were to die in a car accident driving one of his '70s muscle cars next month, WWE would quickly transition into featuring Sheamus as the top babyface of the company and then take anywhere between a year and 18 or so months to establish who the new top star of WWE is. Booking plays an inordinately huge role in all of this. As does marketing and, simply put, propaganda. Back in mid-2010, if Cena were injured to the point where he could no longer wrestle, WWE would have rallied behind Orton and declared he was the top man, the undeniable top star in WWE. It wouldn't have been true, really, but they would have run with it as hard and as strongly and as persistently as possible, until the audience gradually became conditioned into accepting it.
As far as supernova-sized megastars like Hogan, Austin and Rock... Don't hold your breath waiting. WWE's proven that they are firstly entirely content with the status quo they feature today and secondly are in no position to harbor the meteor-like smash of a new star of that caliber. One day it will probably happen but it's a bit far off into the future, at least.