I hope the rust monsters come back. I've had some fun with those over the years. (As just one example, and completely as a shock to me at the time, my group decided to seek them out. Not to kill, but to capture. Their nation was being invaded by a nation using iron war golems, (not classical golems, more a fantasy homage to battlemechs), so they went hunting to capture some rust monsters and unleash them in the enemy camp when the pilots slept. Led to some interesting encounters, I must say. Also one of the most creative solutions I've ever seen.)