I have a very, very large city (over 2 million people) in my campaign world, and I've been contemplating developing at least two new styles of magic. The first is a type taught by the large and powerful thieves' guild that enhances a thief's ability to bypass security, gain entrance to inaccessible areas, open locks and remain undetected.
The second is urban magic, magic that is particularly useful (and perhaps can only be used) in urban areas. As I imagine it, it would combine some aspects of mind control (for making friends and influencing people), trade magic, and possibly summoning the various creatures that can be found in urban areas, either natural (rats, crows, etc.) or unnatural (fear elementals, dopplegangers, what have you). This can be done currently by cobbling together bits from other colleges, but I want something more unidied philosophically and mechanically.