I'd have to disagree, take the Draenei as an example, they're 'just legion' related so people often say Blizz can't do anything with them until the Legion Expac... but what about Felwood and the Blasted Lands? They could've shown up there easily.
Similarly, the worgen could've been more prominent in Arathi and the Hinterlands combating the Forsaken, or just in the Barrens where the Alliance had difficulty with the rough landscape.
Similarly the gnomes, like the goblins, could show up anywhere their tech is used, yes there might not be any grand overarching plotline but they're not 'out of place' in other areas.