A lot of playable races can be mages, but there are some that have the potential to be incredibly devastating with arcane magic (fire and frost included) due to either their physiology, culture, or otherwise. What race do you think has the most potential (cue the human potential comments) to cause widespread destruction?
For me, Humans, Draenei, Nightborne, Blood Elves, Void Elves and even Night Elves (Shen'dralar) all come to mind, but I'd argue that Nightborne take the cake because of their biological transformation and their societal shift toward implementing arcane magic into the fabric of their lifestyle. I think you could make a good case for Draenei/Eredar, though, because they had an impressive magic-based society on Argus prior to their exodus. What do you think?