I like the idea of UFO sightings. UFO extender (I think) had an option where UFOs could randomly crash while on a mission - that was a nice idea, too: some founding nation calls you to investigate a UFO crash in their territory.
maybe we could include both options as probability values:
UFOCrashProbability - for every undetected UFO, every hour, game checks against this, if check successful, UFO crashes and crashsite warning pops up
UFOSightingProbability - for every undetected UFO, every hour, game checks against this, if check successful, pop-up notifies you "UFO has been sighted over %country"
what do you think?