I was thinking of an interesting idea open world games can use. While your walking around you can listen in on npc's conversations. But depending on the conversation it could unlock a special random mission.
Example: Say 2 npc's are talking about a child who went missing near the local store. This will generate an event near the store where you can investigate and find clues in the area to find out where the child is and who abducted them. You then go to the location and kill the people who are holding the child before taking them to their family.
This could truly add a sense of non-linearity to games!
What do you think?