1. Not sure how cheap this is in terms of either coding time or cpu cycles, but I have a suggestion for countering the wall-off-a-colony-inside-a-mountain strategy: if a colonist doesn't have any available path to the edge of the map through a door or open space, the colonist receives a -10 happiness buff that lasts at least as long as there is no path. Maybe call it a "Feels Trapped" thought.
2. It would make sense if prisoners would occasionally attempt to escape by turning hostile and making a run for the edge of the map. Low happiness, moderate fear, and high health would make sense as the most likely precondition for attempting to escape. Escape attempt events by groups of prisoners are the next logical step.
Edit: Almost forgot! 3. An event in which a novel plant lands on the map, spreads geometrically, and must be destroyed with fire, explosives, or simply by time-consuming plant-cutting. Possible effects of the plant: turning areas of fertile terrain into infertile terrain, area damage effects (i.e. anything walking through it gets hurt), negative psychological effects within a certain radius of the plant.