Rise of the Hutt Cartel launched quite awhile ago now, and there have since been two "mini-expansions" that are more focused one expanding the game's features than adding additional leveling and story content. Galactic Starfighter gave us PvP space battles, and Galactic Strongholds will give us Player Housing. Galactic Strongholds is billed as being "3.0", but as far as I know, there's no level cap increase or new planets (at least not with launch), else they'd have announced it already.
After just now finally starting to play through the Maekeb story myself, I have one theory on why they've decided to hold off on raising the level cap again and giving us new planets.
One of the most defining features of SWTOR's leveling (at least for me) is the class storyline. Each class has a feel and personality all its own, and your dialogue options reflect that.
Unfortunately, for RotHC they made the decision to merge the storylines and just have a Republic and Empire version. Ilum kind of made sense as an "endgame" planet where people could go for "something to do" after reaching 50, be it PvP, dailies, or whatever, but once you set foot on Maekeb you leave your class story far behind, and the stuff you do is pretty much just basic military operations. Not that it's a "bad" story or anything, it just doesn't have the "personality" that the class storylines have. It makes me think that the most "canon" classes for everything that isn't class-specific (sidequests, RotHC story, Galactic Starfighter, and so forth) are the Trooper or Smuggler for Republic, and Imperial Agent for Empire.
After all, why would Darth Nox waste his time and energy dealing with these pesky Republic soldiers when Imperial Intelligence is so eager to handle the job? Why would the Jedi Council risk losing a valuable member like the Barsenthor to the dangerous potential threat that is the Hutt Cartel when a healthy supply of cannon fodder is ready to volunteer in his place?
I think it would have been much better if they'd given all of the classes some reason to be there, but still kept some of the more interesting missions as sidequests.
But if Bioware decided they felt the same way and are going to extend the class storylines at some point, it'll be no easy task of course, so that might be a possible reason why they're holding off in favor of expanding the amount of activities at endgame.