My dream expansion ideas would be called "The Outlaws" - crime and punishment, outlaw bases, etc... and "The Explorers" - or maybe "The Surveyors" being an exploration overhaul, with surface sampling missions, scanner minigames, something?
What I would really want to see would be an ingame encyclopedia/wiki full of lore, people, timelines, and such - kind of a Hitchhiker's Guide kind of thing.
Another thing I would like to see would be businesses. Not just Sirius Corp, but smaller businesses who would provide quick hauling missions - "Hey, I need you to get this load of meat from here to the Bob and Mary's Restaurant warehouse." This kind of thing doesn't need to be related to factions, it is filler to make the universe feel a little more alive. And they need ads on billboards like in GTA - could be sourced from the community.