Personally i think that the Hightowers would be bad choice for a new faction. They are the best candidate for it in the Reach; But it is stated that they are known for being loyal and stalwart vassals of the Tyrells and the Gardeners before them. Also it is stated that they seldom took part in the wars of the realm.
Instead i`d consider Redwyne or Manderly. Manderly could actually be a faction on their own and if they get wiped out the current event for the Starks happens or something.