I've just had this idea (and bear in mind i have no knowledge of scripting or anything to do with modding-- so i am not sure how impossible it would be to implement). The idea stems from two things i find lacking in RTW-- Sieges are too repetitious and there is not much treacherous behaviour (other than bribing).
The idea is this: Upon besieging a city there is a % chance that someone will (like a spy) open the gates for you. The % chance could be related to the besieging army and besieged army's strength ratings-- the higher chance of treachery the more the attacking army dominates the defending army. The % could also increase the longer the seige drags on.

Just thought this might spice those sieges up a bit. Any thoughts?