I'm another that hankers for a return to Group C, but must admit the wet blanket of reality as put so eloquently by isynge, zac, Dutton and others does need consideration.
I must also admit that in the end it did appear as though Group C should really have been renamed Group Porsche and that domination caused its demise. The problem is that the same thing is close to happening with Audi - perceptually at least.
So to sum up, my answer is that I have no answer.
I would resist removing open cars from P1 in favour of closed cockpits. For me, it should always be about variety. Variety of types of cars in a class and variety of manufacturers.
Neil