How cn the BG story line go foreward? The main character is 30+ level and either relinquished his or her godhood or became a god. There is no reasonable way the story can go foreward. Also don't go into that prequel crap, Volourn.
Continuing the Bhaalspawn story would be a very bad idea. Sure, it can be done, but why? There is no point to continue a story line that has ended. I rather see a brand new original storyline set in the city of Baldur's Gate. I think that could be very fun to return to the city if done properly.