If you combine their initial license progression with their Revenant Wings classes (where applicable) and their depictions in various guides/media you might come up with something like:

Fran : Red Mage (black & white magic) + a hunter/archer (bows)

Balthier : Hunter/archer/skypirate (guns and technics, light armor)

Basch : Paladin (white & green magic + heavy armor, shields, swords, greatswords, axes & hammers, bows)

Ashe : Time Mage (time magic) + Paladin (white & green magic) your choice between heavy or cloth armors, magic enhancing weapons or a sword & shield

Penelo : Classic White Mage (white & green magic, cloth armor, rods, possibly staves early on or maces where appropriate or when your tactics dictate)

Vaan : Thief (daggers, technics, light armor, possibly expanding into the upper left weapons quadrant to grab crossbows and ninja swords with poles where necessary)

Vaan is the ultimate utility character because he is good or great in every statistical category, and he also could be envisioned to undergo a great degree of evolution and maturation in his combat abilities from a street thief to skypirate. A bit of magic (black, time, and arcane supplements the party quite a bit under the above class setup) could even be justified IMO.

This class setup lacks in Arcane as well as lagging in Time and in a strong Black magic caster, so letting Ashe or Fran or even Penelo into the Arcane (or Time and Black) magic licenses could help expand your combat options.

If you want Fran to be more of a fighter primarily, with White and Black magic as support, equip her with heavy armor and think of giving her spears like a dragoon or something (since that category might be underutilized). If you are using her for her magic, light armor and bows IMO.

Without magic Balthier loses some usefulness as the game wears on, but you can use him as a stealer/poacher and it might be fun to try to work overlooked technics into your combat strategies.