So far, jobs have all stuck to being pretty traditional - tanks are armored physical fighters, healers are all magic-based, etc. But I'm hyped on the idea of a possible Dancer job that heals through dances, and I'm really hoping we start getting more jobs that break from the established traditions of the current jobs.
Mystic Knight - We don't have a magic casting tank yet, and a lot of people wanted RDM to incorporate some of FFXI's weapon enchantments. Mystic Knight could be both.
Their main weapon could be a mace (Red Battlemage style) with a magic shield offhand - preferably as a single item. Their combo skills could be elementally-aspected mace spell-strikes with short cast times. You would cast the spell, then the animation would be a mace strike with magical particle effects.
Their tank and DPS stances could alter the alignment of their attacks. In tank stance they're white-aligned, and their attacks use earth, air, and holy elements for their effects. Increased enmity on everything and debuffs on combo finishers. In DPS stance, they switch to being black-aligned. Their attacks all switch to being fire, lightning, and ice elements with bonus damage, and self-buffs on combo finishers. The attack animations would be the same, but the spell effects would change. Essentially they're positioned as a buff/debuff tank, with the job gimmick of swapping weapon enchantments.
Chemist - A non-magical medicinal healer. Their main hand could be potions or a potion-launching small cannon thing. Their main healing skills could be Mend and Medicate, with damage skills like Acid and Volatile.
Their job gauge could be a potion beaker, and as you do different basic actions you add "ingredients" to the beaker. When it fills, you get a party buff skill depending on the ingredients you put in. The gauge would change color as it fills to show what buff you're leaning towards. If you fill it through mostly healing, it turns green and gives Regen. If you fill it through mostly damage, it turns orange and gives a damage buff. If filled with mostly buffs it'll turn blue and give Refresh. For added flair make all their skill icons match these same colors - green for healing skills, orange or damage, etc.
Synergist - The idea here is to make a Magitek-based caster instead of an aether-based caster. Their main head weapon would be some kind of Magitek focus. To fit with their name their job mechanic revolves around the way their spells interact with each other. I'd give them spell combos, since no caster so far has combos. When a combo is completed, they get a charge that will then alter the way another combo works. For example if you use Combo A first, then Combo B will become an AoE. Or if you use Combo B first, then Combo A would apply dots, etc. Instead of having an "optimal" combo rotation like most jobs, you would change the order you use them in for different effects.