Here's what I want. I have my own opinions on how they could work. These are the two roles I think would be next given the jobs from the last expansion. And ranged physical and magical dps are the only ones with 3 full jobs and it would balance them with the healers and tanks 4.
Ranged Physical DPS: Beast Master, that shares gear with Ninja and wields whips. They are part of FF lore especially in 14 and have used that weapon in the past. If it were up to me, I would model the pet system after the companion system that's already in place and expand upon it. They have a base system that would be easy to upgrade to integrate into a job so they dont need to reinvent the wheel.
Magical DPS: I would bring in the time mage. Looking at the magical jobs, there's a really only 1 form of ff magic thats not really there. Time. Adding in haste, slow, demi, etc would be fun. Maybe have them wield hammers and be a mix of ranged and melee dps, similar to the rdm but maybe focus more on melee.
Then I would add Chemist for the next healer: kinda like RIkku's Mix ability in ffx. and tank I would add Sentinel or Berserker to bring Heals and Tanks to 5 to match melee.