no they wouldnt all those sub races are already allied or have factions allied with the respective faction with the exception of felblood elves and wretched (maybe forest trolls depending on whether blizzard still counts the revantusk tribe)
the majority of the work comes with either developing a new model or making armor work on the current models they have.
some require less work than others but if they decide to do subraces its not like they have to do them all at once and they can have them be unlocked in different ways.