First of all congratulations to reach final round interviews!
While first round interviews are highly standardized especially at McKinsey, in second-round interviewers bascially everything can happen.
The main idea of second/final round interviews is to ensure your performance in those areas in which you did not convince McKinsey in the first round interviews. This can be either the case interview or the McKinsey PEI, or both. Whatever it is, your weak points from first round interviews are where you can expect a strong focus in the second/final round interviews.
So instead of worrying or speculating about out-of-the-box questions, I would rather try to focus on the "standard" stuff instead - this is where you can have an influence, about potential out-of-the-box question is esentially poking around with a stick in the dark..
Hope that helps as a more general idea - if so, please give it a thumbs-up with the green upvote button below!