I\'d like to make something like this:
10.1.1.0/24 10.1.2.0/24
+------------+ +------------+ +------------+
| |
It is kinda crazy to do this, but it's not a bad way to spend your time. You'll learn a bunch of interesting stuff. However you might want to think about hooking the packets a little lower - I don't think scapy is capable of actually intercepting packets - all libpcap does is set you promisc and let you see everything, so you and the kernel are both getting the same stuff. If you're turning around and resending it, thats likely the cause of your packet storm.
However, you could set up some creative firewall rules that partition each interface off from each-other and hand the packets around that way, or use something like divert sockets to actually thieve the packets away from the kernel so you can have your way with them.