My thoughts for Warlock, admittedly a bit bias, would be more of a buff/debuff style class. I absolutely LOVED Mesmer in Guild Wars 1, as it was truly a revolutionary type of class in that style of game. The anti-class. I would really love to see something centered around causing damage based on actions of the mobs, buffs based on my actions, and the interaction between those. A complex class, difficult to master, but strong if you can. The buff/debuff style would also make it attractive for party play.
I’ve been aching for another game with a class like that, even Guild Wars 2 failed in the new iteration of Mesmer.