I have one Mindstorms set and soon will get another. One idea I had for a robot is an outwardly useless one, but presents a challenge: I want to create multiple robots, who can assemble themselves into a larger robot, Voltron-like.
I'm not sure how I'd set the bots up so they could recognize the other bots and dock with them. RFID sensors?
The different parts could communicate, once they are assembled, by servos pressing the buttons on the NXT bricks.
And what would it do once it assembles? Hard to say.