The only thing they can do is create expansion quests that will be impossible to beat solo. The major problem will be hacked characters going online as they are saved on private pc's instead of a server maintained by Larian.

Now if there is a summoning skill that a 2nd player can take the role of the summoned creature it be rather interesting.

Every time there I run into trouble on the road, there is always a dwarf at the bottom of it. Don't they know how to drive above ground?