So after seeing how they set the new jobs i could easily forsee an addition of dual classes.
We already have the DRK that is a DPS Tank but can only que as a Tank. Maybe someone would rather dps sometime.
So with that i was thinking back to my FF11 days and thought about 2 classes i would live added and would be perfect dual que classes.

The Samurai - a dual weilding Tank or DPS class.

The Red Mage - a ranged caster or caster healer class.

I personally loved the Samurai from FF11 and when the Ninja was added it brought back my fond memories of the Samurai. If it plays close to how it did in 11 i think this could be a really fun job to play.

As for red mage having a strong focus of direct damage spells and some dot links to maximize its eeffect as dps and then for healing i envisioned it using more of a HoT play style that is nothing like what we have in the game right now.