I think tanks and healers are covered for now and more than enough...I think since they are only doing probably two jobs in the next expansion they will both be DPS.
1. Red Mage or Blue Mage...probably Red Mage first because Yoshi-P just doesn't ever seem confident when he talks about it so Red Mage Melee Caster DPS.
2. SAM will be melee dps.
I think adding another healer or tank right now would just cause more issues with groups and I think they know that and will just give dps more choices for a while.