Just make them a sword using class with access to all whm and blm spells through cross class. Let them choose their own abilities through that to determine if they want to focus on stronger heals or stronger dmg spells.

Their traits could give access to astral fire and umbral ice if they have a fire and ice spell cross classes, ect. Or free cures if they take cure 1 and cure 2.

The details? I'm not thinking on it to much atm, but the idea behind making a job that utilizes the cross class system sounds awesome and rdm would make the most sense out of it.