Hmm, I kind of was thinking that THM would get WHM, I think it would fit better with the existing lore, but those shiny new CON spells indicate otherwise. ;_; In the CON quests (and as seen in the Gridania storyline) the whole thing with the elementals is a pretty big deal, even a central one I daresay. And here are some copy/pasted tidbits from the NPC's at the THM guild:

Tutubuki: If you would walk these halls, you must know of Nald'thal, the two aspects. It is Nald who rules over this world of the living, and Thal who keeps the realm of the dead.

Fyrilsunn: Arrzaneth Ossuary [the THM guild in Ul'dah] is the temple of Thal. His aspect is that of death and darkness, that which we shall all come to know in time. Many of the poor and destitute worship here. For it is they who seek a better place in death than they knew in life.

Totono: East of here lies Milvaneth Sacrarium. Within its halls, the exaltations and praises of life are sung. The aspect of Nald governs this side of existence─passions and possessions. Many who worship there are the wealthy and privileged─those most grateful for what they have.
Based on that I'd have thought they could just give THM a quest to join the presumably more exclusive Milvaneth Sacrarium as their quest to unlock the WHM job.

Though I agree with a lot of posters here that neither mage job can neatly fit in WHM or BLM, and arguments can be made either way, so its gonna be sort of messy no matter what they do. xD Ah well, maybe they are planning something totally unexpected, who knows!