More grasping at straws to turn Cena heelz and give him a mouthpiece manager (what, what, Norcal?), but I'll actually bite this once. I normally just come in and shit on the idea of turning Cena heel...but I'll be nice today. I think it's rather becoming of me, actually.
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That would be one way to turn Cena heel, yes. However...you're over-thinking it. Cena is the champion now. Think back to who was cast out when he was champion. I'll give you a hint. He's currently on Raw. He's main-eventing. He just lost the title to Cena.
Randy Orton.
Orton was cast out because of Triple H's jealousy over his WHC title win, as most people probably know. Orton got all mad and started feuding against Evolution, blah blah blah.
The point of the call-back is that Orton was turned out because of this title. Have Cena join DX, be happy and go lucky for 2 months or so, then have Trips starting to get ancy over the title. Jealous. Finally, Triple H kicks Cena out of DX, claiming some bullshit.
This is where it gets tricky. If Cena gets all good guy indignant, Cena stays face and Trips goes heel, most likely taking Shawn Michaels with him. Have Cena go crazy, Triple H and HBK calling foul and acting confused, and bam. Cena is now a heel, and Trips and HBK are relatively clean in the whole fiasco.
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..Not that it'll never happen. Cena is a face, and a damn good one at that. He's staying this way.