Playable Garleans wouldn't fit in the story without making special changes for them.
I don't really think the comparison to jobs really works either. The game pushes parties away from duplicating jobs, so in any battle content there will probably only ever be one white mage/azure dragoon/summoner/whatever anyway, and in the open world it's easy enough to dismiss others with your special job as using similar but still distinct disciplines.
You are the black mage, those other people are just very advanced thaumaturges or sorcerers/sorceresses.
You can't really hand-wave away a bunch of
super special magic using good guy Garleans quite as easily.
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That said, a PvP mode or something where we could use that "role-playing" system they sometimes bust out to play as a Garlean with a variety of Garlean mini-jobs to pick from would be very cool IMHO. I do find the Garleans interesting, and have wanted some Garlemald inspired jobs for ages, was hoping this coming expansion would be the one but I guess that dream's dead in the water

.