But that doesn't mean Night Elves lack the capacity to be mages. There's a profound difference there.
I'd have a hard time believing Humans, Undead or Blood Elves have the capacity to be druids or shaman.
The only equitable thing to Night Elf mages are Draenei Warlocks... and frankly, the demons and fel powers are a lot more of a sore spot with Draenei than arcane magics are with Night Elves.
If they're going to be adding class/race combos, they should add Worgen paladins (no reason a Worgen couldn't be trained as a paladin,) Draenei Rogues (the Rangari are basically rogues,) and gnome hunters, because they're the last class that can't be hunters, and if goblins can manage a pet I'm sure gnomes can.