Interesting video from LawVS this morning about FOM setting up a "Strategic Reserve" of tracks that could be drop-in replacements if an event cannot be run as scheduled for whatever reason. There is some concern that Qatar and Abu Dhabi may not be able to be run if the shenanigans in the Persian Gulf extend through the Summer and as the four Middle East tracks make up a significant amount of the hosting fees paid to FOM, losing all four with no replacements will mean not only a haircut for the FOM, but also for the teams (whose receive a percentage of those fees as part of their annual payout).
The two most-obvious choices would be Istanbul Park and the Algarve International Circuit as both are formally returning to the calendar in 2027 so they are in the best position to be readied for the end of 2026. And both would have suitable winter weather to host an event compared to tracks more north in Europe (for example, Silverstone).
Another option could be the Sepang International Circuit, which has maintained an FIA Grade 1 status since it left the calendar. While the Malaysian government continues to feel that the returns from hosting an annual Grand Prix do not cover the costs, a one-off event at the end of this year with a significantly-reduced hosting fee might convince them to allow it. The circuit is also formally sponsored by the state oil company PETRONAS, so there is a direct tie-in with the PETRONAS-Mercedes AMG team (who will likely have secured both the World Drivers Championship and Constructor's Championship by then). And as it would be Summer in the Southern Hemisphere, weather is not a concern (and hey, the heavy rains are a feature!).
A possible wild-card would be Thailand, which is very motivated to join The Circus as soon as 2028. While the current plan for the proposed F1 Grand Prix is a street race held in a suburb of Bangkok and therefore not an option for 2026, the Buriram International Circuit does hold both FIA and FIM Grade 1 status as it is the home of the Thai MotoGP race.