At the moment, Rey Mysterio and Sin Cara are recovering from knee and ACL injuries, respectively.
As a result, Mysterio hasn't been on WWE TV since August 2011 and Cara has been sidelined since Survivor Series 2011.
Injuries or not, the WWE may schedule the two high-flyers for WrestleMania 28 even if it's in a non-wrestling capacity.
According to the Wrestling Observer Newsletter (via 411Mania.com), Cara and Mysterio might be in Miami by the time April 1 rolls around:
While it's not known whether Sin Cara or Rey Mysterio will be able to perform at WrestleMania 28, WWE is still pondering whether to have them appear so they can get the world record for most people wearing masks at once in the same place. The original plan was to break that record via a feud and match, but it has not been possible.
Unless Mysterio and Cara returned next week on Raw or SmackDown, it's unlikely that the WWE will have enough time to plan a worthwhile feud between the two men.
Even if the two were back in time, it would be a disservice to the fans to rush the marquee match. As highly anticipated as it is, it's better to do it right than to bungle it like so many other haphazard feuds.
Not to mention, because the WrestleMania 28 card is already stacked, the decision to add Mysterio vs. Cara would probably not make a significant difference business-wise.
Ideally, the bout should be saved for a future date—like SummerSlam in Los Angeles—where it would not only receive the attention it deserves, but make a tangible impact.
Still, if the world record for masks is such a priority, there's nothing wrong with advertising a "confrontation" between the Mexican stars for WrestleMania.
Alternatively, if they can do a few spots in the ring then they could be programmed into the John Laurinaitis-Teddy Long feud that will likely culminate in a Raw vs. SmackDown melee at Sun Life Stadium.