Let's not re-invent the wheel (again)! Firewall builder does this already.
I tried fwbuilder for awhile and its level of abstraction made it difficult to see what was going on at the iptables layer. From the fwbuilder FAQ:
3.1. Is there any way to import iptables (or ipfilter, pf, ipfw or PIX) rules to Firewall Builder? No, currently there is no way to import existing firewall configuration into Firewall Builder
Does fwbuilder even know how to use any of the more interesting and exotic iptables modules? Or does it only handle stuff that it can also use for the other OS's that it supports?
BTW, as long as I'm in brainstorm mode, a GUI front end for the traffic shaper system (http://lartc.org/) would also be nice. Or at least initscripts to manage it.