What about adding Tarbeck Hall and Castamere keeps in the Westerlands? These should be under the control of lord Tywin Lannister at the start.
Adding them could be interesting if someone wants to roleplay as an exiled member of House Tarbeck or House Reyne, trying to regain the ancestral seat of their house... as a plus you could add minstrels around the westerlands singing the 'Rains of Castamere'
By the way, why did you choose to unite Westerlands and Crownlands in an unique faction? Wouldn't be better to have them separated, one lead by Tywin and the other one (if not by Joffrey) by Cersei.