If/when FSU leaves, the Seminoles and Clemson would likely be SEC-bound. UNC and UVA, which both have AAU accreditation, would join the Big Ten as basketball and academic fits. The Big 12 would likely try to poach Louisville, NC State, VA Tech, Pitt, and/or GA Tech.
There will be a big game of musical chairs played when the ACC crumbles, but unlike the Pac-12, there is a contingency plan to keep the ACC alive: Memphis, Tulane, Wazzu, Oregon State, UConn, and USF making the jump from the Group of Five ranks.
Enough ACC schools likely won't get invites to the other conferences since there's only so much revenue split current members will want to take part in, so with the Pac-2 schools and AAC programs making the jump, survival would be more than possible. The American would simply pluck programs from the Sun Belt and Conference USA, and those conferences would fold FCS schools that qualify for FBS membership into their ranks.
LINK: https://flywareagle.com/posts/acc-ensure-survival-having-at-least-15-schools-fsu-clemson-leave