Source: The Wrestling Observer Newsletter
WWE is planning to have Rey Mysterio play the heel in the Mysterio/Sin Cara feud leading into WrestleMania. Some in management are resisting the idea but the feeling is that while Mysterio still moves a lot of merchandise, Sin Cara has a better chance of getting over as a babyface between the two.
As noted, there are plans to have the two form a tag-team and possibly win the titles. That would then lead to an eventual split leading to a big singles match between the two at WrestleMania 29.