Is there enough room to have it control the ethernet port for another weaker or perhaps more powerful microcontroller?
Can you combine multiple picos with one being the ethernet stack and another that modifies certain packets?
Are there any other interesting things that can be done?
Well there is a whole unused core and plenty of built in SRAM. Seems like a good way to have an open-source version of Wiznet chips [1]. It could support full protocol offloading like Wiznet's or a lower-level raw packet sender/receiver like the ENC424J600.
I don't have the time to give it a shot myself, but I could try to help if needed.
Back when I was doing a dumb-server/smart-client desktop environment. Something like this would have been pretty cool. It needed a tiny API to save files, but the bulk of the environment worked as a static server.