Let me get this straight, you are looking for a Meg/Gig quota limit, so when the user reaches say 1.5Gig out of 2.0Gig total allowed download per month they would be warned, and when they hit the 2.0Gig limit, they would either be cut off or charged extra -- Something like that? snmp, mrtg/rrdtool would be your friends in this case, I'd say...
Bandwidth Limiting speed wise would also be nice, if that hasn't been done, trickled is an easy to use program that has a few options to limit by port, by ip, and so forth. With something similar you could have per user speed options also.
There are hundreds of things that could be done with an ISP configuration package, just a matter of what people want most, and the time to make it happen.
Appreciate the input, and I'll do my best to pass it along...