Blue mage would be interesting, Green mage as well. Honestly, the concept behind Red Mage seems to be more liked a gimped version of SCH. A Good SCH will be DPSing, and healing at the same time (Unless the incoming damage is extreme, like turn 4, Phase 5 of titan ect) because we have a fairy for healing. Time mage could possibly prove interesting, so could Elementalist.

A lot of possibilities, the challenge comes with balance and role placement. Duty finder makes this hard because everything must be a tank, healer, or dps. No room for support at all without filling a previous role.

But to be honest, I could see a Tank / Support class. Support classes do exist, BRD for example is a DPS / Support, and SCH is a DPS / Healer / Support / (Tank if you are crazy, I am).