I'm hoping for a abyssal maw raid so that we can fight raid bosses such as Ozumat and Neptulon. Sooner or later we need a underwater instance.
But an Uldum raid would also be very welcome. Lorewise, I was a little disappointed in Halls of Origination because there was barely any lore about the titans there. I hope a new Uldum raid would be as EPIC as Ulduar was. Ulduar had really good music, bosses, design and lore.