The first test will be ESPN's ability to launch the ACC linear Network in 2019. Followed by the Big10 TV negotiations, which may include scenarios with increased TV rights value if UT, OU, etc would be added.
That deal is up for renewal in 2023, but the Big 12 GOR will still run through 2025. That Big 10 deal is going to be multi-year and worth BILLIONS of dollars. Would TV partners really be up for renegotiating new TV contracts 2 years later in order to add 2 (or more) Big 12 schools? The language would also be spelled out for any potential expansion candidates on what would happen if they were to join the Big 10 (such as, unequal distribution until so many years in the league) - would that really sit well in Austin?
Best case scenario is still the same - demand an extension on the current GOR and just stay with it until it expires.