Descent started out as a single box that _grew_, and eventually encompassed a campaign. Then Descent 2nd edition came out and made sure all the great ideas were included in the first box- or at least allowed for. They also sold a conversation kit to turn Descent 1st Ed into 2nd Ed.
I kinda hope X-Wing goes the same way. Give us an X-Wing campaign, and a new rule set that encompasses Huge ships from the beginning. I am, frankly, jealous of the fact that Imperial Assault has a campaign from the beginning.