Well Pestilence and Moulder could easily go their own route with the grotesqueness of either side.
So easily expandable.
While the techies and Mors could be more or less unified to depict a more generic Skavenforce.
While leaderchoice, batallions and some other nicknacks would allow one to still make something part of either Clan.
Still a single tome would mean it's more likely to be updated.