I just honestly can't come up with 2 school that could join the B12 and make the conference more attractive nationally in Football, short of something totally ridiculous like BYU & Notre Dame.
The B12 isn't going to be able to poach anyone from another P5 conference. Another Texas school(Houston) adds no value to the conference, the schools already in the B12 + Texas A&M(who isn't coming back) has to account for about 95% of the football fans in Texas already. So that leaves someone like Colorado St, but can they really bring the Denver TC marker? Not very much of it I would think. Denver is a pro-sports town, it's never really cared about college sports as a whole. Colorado St is twice as far(at least) from Denver as the U of Colorado, and they aren't exactly tearing it up in the fan support department.
So then you can push East, but are schools like Cincinnati and UCONN bringing much value in the TV market? Cinci has been alright, but not overly great in Football for the last decade since Brian Kelly left and Uconn has had some decent years in recent memory, but holy hell that school would be quite the outlier in the conference. A lot of people are already bitching about having to travel to West Virginia.