I've seen various rumours that BG3 would be produced - however nothing substantial (yet
).
However - I think a major factor would be that they would have to start with a new character - after all the ToB level was just insane, and the endings kind of spelled the end of that character.
So if another BG has to be made - it would be a new character - but what about storyline?
You couldn't play another Bhaalspawn, because after all - that saga ended with the character in BG2-ToB. Otherwise you have to play a bhaalspawn that doesn't care about his heritage and isn't influenced by it (doubtful according to the BG-series storyline).
The only way I see a BG3 being produced would be with a brand new storyline, trying to capture some of the "fame" from the name Bladur's Gate. I have a hard time seeing the usage of a BG3 as more then a marketing strategy. (Much like I fear Fallout 3 is being used by Bethesda
, but that is a different matter).