I think you would need to do the DLC as not so much an expansion but another game. Do a DLC that is just a new campaign, just the story needs to be done, the assets for the engine could stay the same. You again start at level 1 and then go through the new campaign. In this manner the game could have multiple "DLC" provided they can keep the story plots well done. Each could be stand alone and so less pressure for people to purchase any or all of them.
