All the charity work he does, and the fact that he appeals to women and children, Cena will never turn heel.
WWE doesn't want to risk losing it's positive PR
guy, and the younger demographics in this PG era.
Also, since the WWF bought out all its competition, they have no reason to 'try' anymore. If WCW was still nipping at their heels, or surpassing them, WWE would be FORCED to think outside the box. Vince monopolized the industry and killed any reason for WWE to try anymore.
That's why current WWE product is relying on former talent to draw. They've run out of ideas, and have no need/reason to build new stars. Just plaster The Rock's name on a marquee and collect the quick money instead of planning on the long term. It's WCW all over again, only this time there's no one to make them pay for it.