I rarely have anything 'external' inspire a mission. I just load up the editor, and look at the terrain in different places. Just drop a playable unit somewhere with an empty 4x4, and start exploring. I see where some interesting operations could be mounted against an area, and then build it.
Sometimes nothing comes to me. I can remember several times driving for an hour or more through every part of an area of the map, where I couldn't drive I hiked. Got nothing. Other times, just minutes of scouting and I've started building the mission.
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