Necromancer will never be in the game - Yoshi P said this himself. Too much censorship in certain regions to make it viable.
Time Mage is in the game - it's Astro. Like it or not, that's the case. Haste's and Slow's will never work in the battle system.
Geomancer is already in the game. It's what they call conjurers in the Far East - it won't be it's own job.

If we see something, it will likely be something new, though it seems to me that DoT's are a dying breed and I wouldn't be surprised if they were all gone next expac.