What do you think?
So far Queen Azshara is confirmed and she is tightly connected to the Old God N´Zoth. Much of BfA also reeks of N´Zoth influence and there is a herald of him in the first raid. So I think we can expect the final boss to be N´Zoth or either someone corrupted by him. Hopefully no Garros vol 2 though (Jaina or Sylvanas). That would be so disappointing.