For shifters: Using the PnP explanation of them
here and the Extra Feats every character gets at level two, the ability to copy forms, if only in appearance. One extra feat used as a form copier, the rest used for the various forms. Perhaps some sort of rest menu addition for the deletion of shapes.
Slightly more drastic: Lessen the requirements of undead, outsider, and construct shape, but delete the forms it gives. Taking these feats allow shifters to copy the forms of undead, outsiders, and constructs.
This might've been buried in the other topic, so while I'm suggesting things.. Allow players to request monster pcs as new characters. They can be a peasant that got turned, or just a wererat that got pulled into the mists, etc.