Great question admin! I would say the most likely to suceed spinoff would be Francis and Piama. First off, with them you have two developed characters which, in my opinion, would be easier to make work than one character, as we saw with the Friends spinoff Joey, where there was only one character that the audience was able to draw off of from their history with him. The audience of MITM knows Francis and Piama and their history, characteristics, flaws etc, thus making for relatable characters. Plus, they are at the right age to begin a family of their own and we could see Francis incorporate his experiences from growing up with Lois and Hal, rub off on his kids. We could see him struggle to decide if he should be more like Lois or Hal with his parenting skills or develop some of his own. It would also be funny to see Francis in a permanent work place and get to know his co-workers for supporting characters. And the occasional visit by Lois, Hal, Dewey, Jamie, Malcolm, Reese and of course, Ida. would ensure that a part of MITM would live on for years to come.