I agree with Fable on this one.
Playing a Bhaal spawn - or even the new Lord of Murder would be a seriously destructive aspect of a possible BG3.
The "Saga" of the Bhaalspawn ended in BG2:ToB, and I think it would be best left there.
As Fable says - to play as a God vs. another God would requier so high level characters and abilities that ... well - Fable said it
And playing as somebody else with Bhaals blood in them would be rather dull... seeing as we kinda knows what happens with (all of??) the spawn.
We know what happens with the most powerful Bhaal-spawn (ourselves in BG2:ToB), so there is little meat to pick on spinning that story.