My choice would be a rune knight or something of that nature. Essentially a melee range caster that uses magic to augment their attacks, maybe even with short cast times and, like the other casters, a reliance on MP.

Second would be basically a proper archer/sniper class for ranged DPS. I know that the devs have said there won't be any more split branch classes like SMN/SCH but I'd still love to see a bow user that branches off from ARC into a more pure DPS focused class, perhaps even borrowing some of the concepts in BLM where you have longer ability activation times and more limited movement but with tools to play around with those challenges.