I'll venture that it won't be a Martial class; I'm just not seeing how the flavor of that would work. And it *is* possible that there will be just one controller class! But, it's no fun to speculate that there won't be one, so here I go
Beyond that, I don't think we even have a candidate--and I think that's deliberate. This one is probably intended to be a 'surprise!' moment later on. If so, then it's probably *not* an 'obvious' choice, like say druid (I'm going to guess that Nature Priest-type Druids are now clerics, and Druids in other modes will come later).
I'm also going to guess that Sorcerer won't show up as an Arcane Controller: with Wizards having lots of at-will and per-encounter powers, the 3.x Sorcerer's purpose evaporates. If we see a Sorc, it'll be noticeably different than the current one (my guess: Arcane Leader).
So what does that leave us? Specialist Wizard-like classes, like Illusionist, Conjurer or Necromancer would all work. Some form of Elemental Manipulator might work, but that sounds more like a fourth power source to me (and I think all the Oriental Adventures style characters, like Shugenga, will be in a later book). A Divine Herald, that summons up the servants of the gods? [Solar, I choose you!
]
Anyway, that's all I can think of...