Re: Rey Mysterio heel turn a possibility against Sin Cara at WM29!
Turning heel is certainly the only thing at this point that could simultaneously refresh Mysterio's character and force Sin Cara into a very favorable position, but it's a huge risk and not something I'm entirely convinced would work.
Mysterio is unquestionably one of the top draws in the company. While Sin Cara is yet to really establish himself, and at this point it's unclear whether he'll be able to become the replacement many expected. Turning Mysterio heel could damage his marketability during what has to be considered the final stretch of his career. I don't know if that's something the WWE wants to lose at a time when they have so few real draws.
Having said that, it could be a very interesting angle if done right. Mysterio has established himself as the quintessential babyface during his run in the 'E. He is quite possibly the most babyface babyface of all time, or at least since The Dragon. That's given his character a lot of length, but it has come at the cost of limiting what he can do. A heel turn would allow a much different Mysterio, and as good as Rey is, that's something that I'm sort of intrigued by.