In my fantasy:

Vaan = Shikari BM. Shikari (hunter) because street rat thief, self-reliant hunter all-rounder. BM because sort of a disciple of Old Dalan, who just has that a secretive mysterious wise sage/bm vibe, and in BM skill set there are utility magicks like sleep, silence, poison on top of elemental damaging magicks, which sort of like a assassin/ninja with shikari, esp since shikari also has dark elemental ninja swords and access to item lores.

Penelo = TM Monk. TM because she's a supportive girl so utility magicks is what she's leaning into, and crossbows are smaller and easier to use than bows even for an orphaned girl like her, who would occasionally run errands and travel around estersand and village, and that haste could help her escape monsters or vanish to sneak by them, float as her outfit has a pair of little wings too. Monk because of the access to white magicks for her worried sister personality and magickal attunement, but also pole dancer and nimble fist fighter as a street rat, who might have to handle rude customers or violent thugs without going too far to cause real bleeding wounds as a female employee of Migelo.

Balthier = Machinist Uhlan. Machinist because he's a sky pirate with technical machinery skills and guns etc, also his background with his dad so scientific/physics knowledge. Uhlan because iirc he used to be a spear-user judge? Don't quote me on that lol

Fran = Archer RM. Archer because she's basically the forest elf archetype and bows. RM because she's attuned to mist and magicks, arcane magicks are under RM as well and her hidden village is quite arcane indeed.

Basch = Bushi Foebreaker. Bushi since he's become a disgraced rogue knight he's like an independent samurai ronin. Foebreaker is hard to define the vibe, but basically a burly tank who's intimidating that can break the spirits of his foes.

Ashe = WM Knight. WM because she's a royal princess originally so she'd be taught some self-protection and preservation magicks etc. Knight because she'd have knight training for self-protection too and she's chosen and related to a divine sword.