That's an interesting thought bubble, but with so many of the game's assets server-side, I doubt it's possible to have an offline option. I'm going to take a wild guess and say that the fundamental design of an MMO vs single-player is dramatically different with both being incompatible with the other.

I think the best they're going to get is making a lot of the game solo-friendly so people who don't want to be forced to have real-time interactions with others (duties, etc) can play "alone among the many".