I could possibly see Marshall considering rejoining the MAC, but none of the others make sense to me. WKU and Middle would be good to have in the MAC, but I can't see them joining a northern league.
I don't see WKU or MTSU wanting to move north either.
It seems like most of the G5 schools fall into the footprint and have some cultural/regional overlap with a P5 conference (and maybe struggle with sharing eyeballs from students with the larger schools in the area). Seems to be some steady non-conference games played between them, too.
MAC-Big 10 (11 of 12 in same footprint)
C-USA-SEC and Big 12 (11 of 14 in same footprint)
Sunbelt-SEC and Big 12 (11 of 12 in same footprint or 9 of 10 football members)
Mountain West-PAC 12 (Only 6 of 12 have a direct overall, but schools like Boise overlap with the Oregon and Washington schools quite a bit etc., New Mexico with the Arizonas (used to be conference rivals).
The American Conference lives up to its name being so spread out, overlapping every P5 but the Pac 12.