We actually have a huge amount of voice work as well as written work waiting, but not part of the game yet. It may wind up being part of the first expansion at this point, because it's a lot of stuff!
Please don't put the story in another expansion. You may take parts of it or an extended lore of it in the expansion but putting up a whole expansion just for the story is a bad move.
This won't give you more sales that you need, trust me. Gamers hate nothing more than DLCs that they consider as part of the main game. And story should be part of the main game, at least the major amount of it.
You may however extent the story into the next game. I still suspect that Rodney will turn out as rogue AI/robot that has been imprisoned there on purpose and in his attempt to flee he led to the whole "stuck in the side of a star" situation.
All the robots you encounter may eaither be inmates or (like the Warden) guardians of the prisoner ship. And Rodney might be the final boss of the expansion. We will see.