Most necessary jobs, IMO:

Blue Mage: Tank. Learns necessary job abilities through defeating certain mobs during class quests. Can learn some bonus abilities through hunting certain monsters.
Dancer: Melee Healer (since no support role in FF14). Heals and buffs through dances. Could utilize TP in addition to MP.

Other than those two, I'd like to see a Corsair/Musketeer. They have teased Musketeer since the beginning and MCH wasn't it. Could have gameplay similar to Corsair in FF11.

I'd also like to see a Gunblade, since they introduced the concept during HW (the Empire dude with a gunblade, can't remember his name). Could be Tank or DPS.