SMU seems the most likely since they’re not getting any money from Tier 1 media rights from the ACC.
I suspect that ACC and maybe the SEC are headed towards differing payouts for the schools, which could lead to defectors depending on how low those payouts dip.
For the ACC, unless their contracts are ironclad they either give FSU and Clemson (maybe UNC too?) more money or they’re leaving. The catch is that to placate their top teams they’ll face losing other teams.
For the SEC, Vandy and others will eventually have to take less. The question is how much less do they take before it causes them to leave. This seems much less likely because the SEC makes so much money that they could tier the payouts and their bottom feeders would still likely make more than another conference could pay.