Also, since this topic is headed External Influences, I thought I would add another suggestion.
Other influences outside the airport could also cause delays or cancellations. I do not know whether this has been mentioned on the forums before, but:
-Weather outside the airport, such as a volcanic eruption, could delay or cancel flights.
-ATC strikes could also do this.
-Delays at airports where flights arrive from could have a knock on effect.
I do not know whether implementing these would be practical or easy. It's just a suggestion really!