I don't know what the Sun Belt plans are. I do remember that solidarity was a word used in the past by Sun Belt AD's. That was about four days before Western Kentucky blindsided everyone by announcing they were leaving.
If I had to make a guess as to potential invitees, I'd name four schools: UTSA (who is about to be the football power in CUSA), Southern Miss (good brand and geography...plus I think they'd jump at the chance), Marshall (if the league wants to expand East....again) and one FCS move up: James Madison who was mentioned just before Coastal was added. (Softball fans would love this one.)
But I'm making the assumption the Sun Belt wants to raid CUSA.