Edited second version!
I’m with Ned. I don’t personally feel all the races should be part of something. I’d probably do:
Monaco (like the Indy 500)
A Cup Series would have to have at least three and the race would have to be on the calendar for the last 10-15 years* or has a history in the sport, or something like that.
*apart from the Classics of course, these would be immortalised forever and no others could join unless, say, Japan and Australia became the only two Pacific countries on the calendar again