Whilst I'm certainly hoping for it, this game unfortunately, doesn't lend itself well for it.
There are such enormous variations in how the game plays out - that it would be extremely dfficult to add a story to it. Whether it plays after or during the main campaign, it would be an enormous untertaking. There is a limit to what people will pay for DLC content, and expanding this by something roughly the size of a single chapter would rapidly exceed the cost of a regular full-sized game.
They did, however, set up a huge infrastructure and they're very apt at using it now, so. Perhaps, more mask-of-the-betrayer-style, where all companions are replaced; a more standalone story? Who knows?
