There's a lot of theories aboud about what's next on the table, now that Ninja is confirmed and a "gun class" is in the works.
However, my prediction is that the gun class (Musketeer/Corsair) will be released alongside...
Red Mage, in 3.0.
If you think about it, RDM would fit the bill perfectly for just about every major decision the design team SE will have to face:
-RDM would fit in the Main Scenario story very well as a character who could step forth claiming to be one of the last of his kind (similar to Yugiri). It would explain why they weren't implemented immediately as well (it may be that there used to be elite soldiers capable of wielding both White and Black magic in the old days, as well as the arts of the sword, but since both arts are outlawed now, they were thought to have died out along with the WHMs and BLMs of eld).
-It's been heavily implied that the mystery "gun class" will do far more than just shoot stuff. Another thread on here has brought up the idea of it being a combat medic/engineer type Job that can heal. It's confirmed that a new Healing class as well as a new Tanking class are in the works for 3.0; if this is true, then it could be that RDM will actually be a Tanking DoM class.
-Details have been sparse, but "Hybrid" classes have been rumored to be in the works, so RDM could be the first example (and test) of such a class. Perhaps it will be capable of Tank as well as melee DPS, while the Gun class can be either ranged DPS or Healer.
-Most importantly, the dev tram has said that they really would LIKE to add RDM to the game at some point, it's just a matter of working it into the armory system somehow. My guess is...that it will use a base class along the lines of Spellblade, or something like that, and the playstyle will be similar to Jedi Shadow/Sith Assassin from SWTOR. He can use melee attacks and a special blade custom designed for him as well as spells like Enfire to boost damage and Blaze Spikes to generate threat, as well as passive Conjury-style healing abilities to survive in light armor.