IMO a convertible only series would be a huge hit, as people would dream of driving these fast convertibles.
Maybe something like FIA GT but only for exotic convertibles. Of course they would need stiff front (windshield) and rear rollbars for safety. It would be great if they require rear rollbar to protude 6 or 8 inches above driver's helmet for safety.
They could also allow windshield retrofitted FIA prototypes.