I'm thinking about hacking an A4000 motherboard into an A500 case. It probably won't even fit with a CPU board on it, but if it does there's going to be trouble.
How about a PC motherboard in the A500 and setup a version of windows that boots into WINUAE. I saw a tutorial that outlines the procedures here.
You'll have a machine with internet, graphics card, (AGA and Picasso96) ehternet, etc.