All class and race combo's exist solely because Blizzard wills it. They can make, or break, the lore for every possible race/class combo. Its just wether they want to or not.
Gnome Druids? Hell yes! Gnomish Engineers can make machines that transform your form and appearance. They could make a machine that lets them turn into animals. A machine powered by the sunlight and moonlight. Tada, Gnome Druid.
Dreanei Warlock. Well, why the hell not? Because its "evil"? They have an affinity to magic, and also for dark magic. Point in case, all those evil eredar. And from having done the starting quests, only a fraction of those space smurfs are peace-loving beatkniks. Alot are far more "practical". And i can totally see some of them embracing dark arts to fight the dark arts. fire with fire so to speak.
It really doesnt take alot for them to get close enough and warrent a certain race/class combo that was formerly against the lore.