I think the Invisibility spell needs a good reworking. Currently, there's no difference of using it or not except at a very specific place and time; at all other times, you are actually talking to people while being invisible because the game doesn't make a difference even in dialogue... let alone the times where it WOULD be expected to have an effect, like sneaking past guards, or the like. (Dunno how you'll handle sneaking past a certain police officer to enter a certain house in the GA route, Cmac, but since many a player will try that, it's worth adressing).
I have to admit that, under a different name, I once made a mod (long useless due to game changes) where the player does what any high-schooler with access to Invisibility spell would instantly do: sneaking into the school's gym girls' dressing room. (Where casting Clairvoyance would find a hidden camera placed by Beasley. Of course. Blackmailing him with the discovery for a free Possession spell optional).