You could do a "fantasy mod" containing elves, orcs, humans, undead... but I'm afraid at the moment having dwarfs and tauren due to size reasons is not yet possible, I think anyway.
It's best to steer clear of any copywrites when making mods as some people have had nasty letters to shut them down on other games.
I think towns as big as stormwind would be too much for this engine to handle.
The new classes would rock though, I'm just wondering how magic can properly be implemented.