I would like to see secret missions or in other words find a dead body, a lone Fallen soldier who has information, or a ship that crashes and a survivor comes out and has a map of where he was going before he dies at your feed. I'm sure this is in the game and if not it will come in the 10 years it has, but just expressing some thoughts.
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I wouldn't be surprised, though random spawn missions like that already have their slot pretty much filled by Random events like the one in the gameplay reveal.