I have always made my feelings known that West Virginia would be the best immediate addition to the Big Ten. I know academically they wouldn't be on par with the others, however, I would imagine and eastern division Ohio St, Penn St, Michigan, and WVU would be awesome on the gridiron. Plus, they have always played smashmouth football similar to the Big Ten.
That said, the acquisition of Missouri would allow the smoothest national transition, in that either Arkansas or Memphis joins the Big XII, if Arkansas, then Memphis probably joins the SEC.
It would be interesting to see what Arkansas would do. They love the SEC for football, but probably feel they would generally perform better in the BXII in hoops. While Memphis would probably be a better fit for the BXII.