Some kind of singleplayer campaign with a nice storyline will attract a lot of new players. Not career mode, but a sequence of missions with good storyline.
It could be even different next missions with different story trees, depending on player's choices and results in the previous mission. Or it could be a collection of quests. Or something else entirely.
The plot can be literally anything, as long as it is interesting enough. And game have a lot of flexibility and resourses to make very complex setups in solar system for each mission, like planetary bases, hidden ships somewhere (which are not shown at the map screen at all and must be found with a tricky quests), some scripted actions with game objects, and many other things - game already supports everything which is needed for campaign.