I've enjoyed white mage and bard classes the most in the game so far. I especially like the party support functions of a bard. I think a class, possibly blue/red mage title, that would be purely party support. The class would not dps, tank, or heal. It would cast buffing dots on party members, either increasing dps attack stats, buffing healers stats, or defensive buffs for tanks. There could even be debuff spells that actually work on bosses such as a % defense or attack weaken. Another great skill to have would be float. Float could be affective for 1-3 seconds to help a player avoid a aoe plume. Putting float on the whole party would be a little much, but being able to cast it on 1 party member would be a great skill that would affect the overall mechanics of the fight.
The new support class limit break could be a party wide massive buff in all stats.