WHM: More elemental spells and more holy-magic. Give them some party buff.
SCH: More fairy skills VS SCH skills but revamp how they are being used and how she reacts. Revamp Selene.
AST: More card related spells (not necessarily buffs but also heals and DPS) but lower draw recast so you spend more time on cards.

Al healers should be able to regen AND shield but they could have separate party buffs that are complementary. DPS output should be similar on all jobs.