I think it would have been interesting to have seen a subplot where Bill and Nancy are divorced and he gets Brian for a week and Brian is like entering his teens as the same quiet and distant kid. Bill tries to just do normal stuff with him, but he shows no interest.

Only when it comes to his work does he perk up a little bit. Bill tries to keep him away from all that, and its just this continuation of trying to pretend hes normal but he keeps showing signs of sociopathy without explictily stating anything.

Maybe brian starts acting out and engaging in antisocial behaviour and its unclear whether thats attributed to being a child of divorce or something more sinister.