First of all, I wouldn’t drop any races. I’d fix the calendar at twenty events, and make the following ten permanent:
Australia, Monaco, Canada, Great Britain, Belgium, Italy, Japan, India and Brazil.
As for the other ten events, I would do what the WRC does and alternate them from year to year. Races that are (geographically) close together – like Korea and China, Abu Dhabi and Bahrain – would be twinned so they alternate between one another. Of course, this would require an extra ten events. Probably the following:
Russia/Bugaria, America/Mexico, Argentina/Venezuela, Germany/Hungary, Korea/China, Abu Dhabi/Bahrain, Spain/Portugal, Malaysia/Singapore, with South Africa and France also being twinned despite being so far apart. And for the final pairing, maybe Turkey and Israel – but it would depend on Israel, Bahrain and Abu Dhabi allowing drivers and teams into their home countries to compete despite having been to the others. But while relations between, say, Israel and Iran are pretty poor, they seem to get along quite well with the Jordanians, so maybe they feel the same way about Bahrain and Abu Dhabi.