I actually think Cena wins the title at Summerslam and carries it to the title defense against Rock at Royal Rumble, where Rock becomes Champion (I don't think he "deserves" it, but I do think it's what will happen). I also see Punk winning the Royal Rumble (the one big accomplishment he hasn't checked off). This sets up a Wrestlemania match between the Rock and Punk for the WWE Championship, which I think Punk will win. Of course, this is all predicated on the idea that Rocky will be at Wrestlemania.
If no Rock at Mania, Cena retains against the Rock and we get Cena-Punk at Wrestlemania, which I still think Punk wins.
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If no Rock at Mania, Cena retains against the Rock and we get Cena-Punk at Wrestlemania, which I still think Punk wins.
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