Witcher games revolve around a very specific individual. But if he was present as the player in your altered variant of witcher 3. Yeah it would still be a witcher game.
Honestly what do you want here? To play as a bhaalspawn? To have the story focus on Bhaal?
Baldurs gate 2 finished the story in the throne of bhaal expansion. It was at its core a rpg focused on the region near baldurs gate. The rules used for the game were (I think. Not entitely sure) the relevant ones at the time. The city itself had little to do with the story outside of beeing the largest city in the first game and basicly the place where the player made his fame.
My expectations: the same will hold true for bg3. The city will play a major role in story reasons and it will probably be the hub where we get the most quests and companions from.
Which brings it pretty much in line with bg1.
