When you send the banner with the last filter they ordered, is that sent on the expiration date of the filter, or is the timing based on your internal campaign schedule? If I purchase a furnace filter today, in 3 months (or whatever the interval is), I'd love a reminder email just about ordering a new filter. I always need the reminders :)
One awesome idea I recently heard of (at the Litmus Live conference actually) was of an insurance company using location data to send a Happy First Snow Day email to new home-owners who recently bought a homeowners policy. It used weather data by zipcode, and was triggered when it snowed in the customer's zipcode for the first time since buying their new house.
If you have location data for our subscribers, or can find a way of getting it, I can see opportunities for one-off awareness-level campaigns around unexpected weather events (make sure your furnace is ready for the blizzard, etc.).