The Scions filled the caster/healer niche for a long time so there wasn't any need for the WoL to be a DoM. It's honestly hard to tell if we will finally separate ourselves from them this next expac but if we do, I would expect the WoL to be some sort of Caster.

Otherwise going down the list:
Pld: Done in EW
War: Done in ARR
Drk: Done in ShB
Gnb: Thancred's poster job
Whm: Cnj Y'shtola's poster job ARR-SB
Sch: Alphinaud's poster job ARR-ShB
Ast: Urianger's poster job
Sge: Alphinaud's poster job
Mnk: Done in SB
Drg: Done in HW
Nin: Possible although sort of covered by Yugiri
Sam: Done in SB
Rpr: Depends if Zero sticks around.
Brd: Ach in 1.0
Mch: Possible
Dnc: Possible
Blm: Y'shtola's poster job
Smn: Possible
Rdm: Alisaie's poster job
Blu: lol get out

If we get 2 new jobs next expac, I expect Krile will job change to one of them (probably new caster job) so then it'll be whatever the other job is which is very hard to guess.

Look at this list again, Mch or Smn would not be a bad choice.