I agree that every fire color choice should infuse more spells with that color, including voidwalker, like you said. The models are already in the game. Instead of outright using a fire elemental, though, is there a shadowflame one, that maybe looks more like the fel elemental? That being said, there should then be a questline for purple fire, too.
This Demonology Warlock is the only one who has the green fire, and the only place where I really notice it, is if I’m in a ground mount-only area, and Felsteed or Dreadsteed are randomly summoned.