They added a healer, tank, and ranged DPS class for the expansion + Melee dps (ninja) was added. The next class is gonna be a mage type class for sure. I wouldn't be surprised if they made Red mage a tank class though, seeing how they made DRK a tank, and ninja a dps.

Red mage is able to carry shields, have spells to debuff the enemy, and spells to boost themselves defensive/offensively for threat.
The different en-spells could have different effects besides the element ie. en-fire increase enmity, en-thunder add dot to targets, en-blizzard adds aoe damage to target, etc.
Phalanx the spell from FFIX would be the damage reduction CD spell
Paralyze/slow/blind would debuff targets aka reducing chances of being hit/being hit less.