You can make Vaan a Bushi to really help out with a certain superboss way out in the future.
Monk pairs well with black mage due to their high HP and access to Holy (requires an Esper), you could also pair with Red Battlemage for Dark-elemental spells and Ardor.
Machinist or Shikari tends to pair well with a white mage, machinist get access to Hastega (via an Esper) and Shikaris have shields, the Main Gauche (high evasion dagger) and fairly high HP. You could also go with Time Battlemage for a fully supportive character.
Knight can pair well with Bushi as well as the Knight gets some White Magicks and the Bushi has a fair amount of Magick lores. Or pair with Time battlemage for some extra Swiftness licenses.
Monks also get access to a lot of Battle lore making them good pairs for the Uhlan or Foebreaker as well.
If it helps, here is my setup as of now:
Basch: Knight/Time Battlemage
Ashe: Black Mage/Monk
Fran: Red Battlemage/Archer (for the burning bow and some extra swiftness)
Penelo: White Mage/Shikari.