I have the firm line of thinking that THM, FIRST JOB, will be the WHM. Jobs are intended, from what I've read, to open new options for a class into a specialized role. Jobs are not intended to replace a class, but open a specialized role for parties.
SE is huge on lore and even more so on their FF series. The THM lore says they worship the twin gods of life and death. The guild hall you currently use for the THM class is their temple of death. On the other side on the city there is another temple, the temple of life. So lore wise, it makes sense for the new job to be WHM.
I hate using a FF11 reference since this is different game, but you look at these two abilities of a FF11 white mage.
Devotion: Sacrifices HP to grant another party member MP.
Martyr: Sacrifices HP to heal another party member double that amount.
Using HP to heal and give MP? The THM does nothing like that, right?
Next, lets look at the FF11 White Mage offensive spells.
Dia
Paralyze
Banish
Slow
Silence
Banishga
Diaga
Banish II
Dia II
Banishga II
Flash
Repose
Holy
Banish III
Do any of these spell look familiar?
Point is; The THM is the offensive/solo base class and White Mage is the healing/support job, thus opening up NEW role for a class like I think a job should. The lore fits, abilities fits, and the purpose of the job fits. If it looks like a duck, and walks like a ducks, and quacks like a duck...