I don't understand that recent "stance" Blizzard has taken onto playable warlocks. Realistically, none of the playable races should have them because no faction would accept them, except maybe undead. So why not allow them to be played by races that certainly have warlocks in abundance. Mag'har definitely should have them. Highmountain taurens would also be an interesting option (Bloodtotem tribe).
Personally, I would go even further and allow draenei to create warlocks, aka become evil eredars. It doesn't make any sense lorewise, but so doesn't the rest 70% of the game until they make a "Chronicle" once every 10 years to retcon and "explain" everything.