To be fair the old Group C was run as an FIA series. I know it all ended in tears but I feel that was as much due to the economic meltdown as the FIA.
The 3.5 litre rules did attract cars from Pug, Merc, Jag, Toyota, Mazda, Lola, Spice plus the BRM, Konrad, Allard, Alfa also had a game, but flawed attack. hardly a series that did not appeal.
In fact more factory teams than the orignial rules and the cars where amazing. seeing a 3.5 Pug or toyota blow by was an experiance.
At that time the US scene also flourished with a different, but close too, set of GTP rules.
Fingers crossed it wont be the disaster many predict, and i doubt it would have happened without some commitment from the current teams.