It could take the Scripted Spells mod (well part of it) and expand on it )and change some parts) a little because he idea of buying such spells seems a little silly. They should be either taught or acquired with different levels IMO (kinda like how House Telvanni teach spells). Example, it could start out you can only become a rat or scrib. There could be a secret clan of shapeshifters that do this.

Also if you shift in public guards could come after you as they think you're evil or something. There could even be a big side-quest with the clan.

Yeah, the Scripted Spells mod is really awesome in some ways but it really could develop in so many other ways too

Check out this race mod: http://planetelderscrolls.gamespy.com/View.php?view=mods.detail&id=5911