Monks in Guild Wars were the dedicated healers, which they've said they aren't doing in GW2. I could still maybe see them doing the class, but probably not in the same vein as XI or whatever does it.
My guess for the last class is some version of
Mesmer, all sittin' there in the back with a dress on and all. Plus it seems to be the one niche (enfeeble/buff specialist) not yet represented.
Personally I'll likely go with a human or Norn main, probably Elementalist or Necromancer from what they've shown thus far n' how they pan out. Probably a magey class of some kind at any rate, given my typical tendencies.