While wandering on a distant planet, flying at 500 meters above the ground, I was telling myself that it would be great to suddenly discover an ancient artefact/alien city ruins, or those kind of thing.
So, I think it could be a very cool feature to have mission like "fly to planet X and find a proof of ancient alien civilization". All the player has to do is flying to the planet and fly near the old city. Of course, planets are real size and it will be very hard to find this city. Maybe a detector could help. But even without a detector, it would be great to know we are flying on a planet where a mystery is hiding ! For me it's a key feature...
Maybe this feature is already in the game, I don't know because the few hours I play with Pioneer was only to fly near planet, without trying a single mission !