If your willing to take up this task, (I'm not holding my breath since you don't use ads yourself) these are some of the features I would like to see.
All ads are tracked, displays, and click throughs.
Support unlimited sponsors.
Editable cost for each edvertiser.
Designate custom banner/button sizes.
Clients have their own login section where they can.
Upload or change their ad
View their ads details
Send their ads to specific IP addresses (canadian, british etc)
Buy more credits
Thats all I can think of right now.
Even if you can't do the payment part, the rest would be helpful.
Edited by Sapien