Ok what I'd like to see WWE do with Sin Cara is stick him in a mini-feud with someone who can work his style and then have Tito Santana (or another Mexican legend if he's not available) come out as Cara's manager.
Cara needs a mouth piece after all. This will keep Cara in the eye of the WWE Universe.
Then I'd purchase the Mistico character. WWE have billions; to purchase the Mistico gimmick/ character couldn't set them back too much. I'd have Santana cut a promo explaining that Cara is going back to his roots and he is going to return as Mistico.
I'd then ship Cara/ Mistico to FCW and have him hone his skills/ work the "WWE Style" for 3-4 months. In the mean time I'd have WWE featuring vignettes and promos hyping the return/ debut of Mistico in the WWE.
I'd have Mistico debut at a big PPV with badass music and pyro/ lighting etc, the whole show. By now Mistico would have decent in-ring "WWE Style" skills, a mouth piece in Santana, and a gimmick/ name everyone knows worldwide (so many people are still calling him Mistico even now).
I'd slowly push him as a face and keep him in juicy fueds with all the mid-card heel until it's time for him to progress to the main event matches (by then he should be botch free!)