Each conference is a collection of like-minded schools with similar cultures and goals. Athletics is only a part of it. The B1G has repeatedly stated what it wants:
1. Large, public universities, preferrably land grant and preferrably state flag ship (11 of 14);
2. Prestigious graduate programs with large externally funded research programs (13 of 14, ex NB);
3. High quality undergraduate programs (14 of 14);
4. Contiguous states (14 of 14);
5. Comprehensive athletic programs with emphasis on student/athletes (14 of 14?);
6. Common culture, essentially Yankeedom and Midlands or Foundry and Breadbasket.
Athletic quality doesn't control membership or expansion. The so-called kings need some princes and princesses to kick around. In a conference of kings only, nobody gets above .500, and nobody goes to the national championship.
TV monies are not determinative of membership either, although they are an important criterion.
Penn State, Nebraska, Maryland and Rutgers all fit the paradigm nearly perfectly. They also are great TV products. Nebraska is somewhat of an outlier academically but not by much.
There are no schools in the ACC or SEC that fit the paradigm except Missouri (AAU), which is a good fit although somewhat southern. Kansas (AAU) is a very good fit. Oklahoma works, too, although it is at a some lower academic level. Also it would reactivate one of the truly great rivalries. That's it.
Notre Dame is a poor fit in nearly every way except football and basketball, and that ship has sailed, especially considering the very great insult of its alliance with the ACC. If ND ever joins a conference completely, it will be the ACC, for which it is a good fit. Most of the ACC schools usually mentioned (UNC et al) are small, have small fan bases and don't draw big TV ratings. The ACC schools normally mentioned are way too southern and have small graduate programs (although often good quality). Texas is a poor fit also, except in football and academics. It is too southern, too far away, a bad partner and too uppity.
So Gordon Gee's preferences remain the most likely, Kansas and Missouri. But any of the three KS, MO NB works.
PS. Purdue and Illinois are founding members of the B1G (tOSU and MSU aren't), and they're not going away. They are weak in football and basketball right now, but they meet every other criterion.