I wanted to point you at two very cool looking tools that Antoine Nguyen has developed, and introduced to me by Patrick Ben Koetter. I haven't had time to look into these in detail, but if you're looking for some fun hacking ideas for Mailman 3, these could fit the bill.
The first is sievelib:
Sieve is an email filtering language specified in RFC 5228. I've always thought it might be a pretty cool way for power users to manage the email Mailman sends them. It might even form the implementational basis for a topics-on-steroids like feature in MM3. This is the first implementation of Sieve written in Python that I'm aware of.
Antoine wrote sievelib as part of his Modoboa project:
which looks like a pretty cool take on making the management of email systems friendlier. Of course, Antoine, Patrick and I immediately thought that it might be cool to explore some integration opportunities between Modoboa and MM3.
For those of you in the USA, what better way to spend the beautiful Memorial Day weekend than sitting in front of your screen hacking on some fun projects? :)