I am trying to tell that when you bring some uniquie flavour to one race+class combo leaving others overboard...(and we all know how Aliance players react when horde get something "cool") well it will bring more anger than fun from the comunity. So if they want create this - they must plan not only race+class combo, but many many of them. At least as promise to create this in future patches. (and this still leave "physical" classes left alone - like warrior, rogue, hunter) wich will lead to more work for the developer team (artist and animation especialy)
About void - well, void is consuming already so in terms of blood and unholy it's easy. And fire - all types of dark fire and ash (ash chains instead ice chains, ash shield and etc.)