Maybe something robotic? We could run Robot OS on a Pi and connect it to 8266es that are driving motors, taking readings, etc.?
Some kind of distributed monitoring. A micro in each plant pot sending back data to the gateway, powered by batteries.
Battery micro with little door or window sensors to send an alert when they get opened and closed
Micros on machinery measuring vibration so you know when to service the machine
Single micro doing real timey stuff whilst the gateway does higher level things i.e. quad copter
sth like a model house, and add a bunch of sensors everywhere, a main gateway somewhere else, and do a small-scale representation of a house full of sensors and a main gateway controlling them