It makes perfect sense that they wouldn't want to do HC anymore. The X6 is being produced out of that plant. It would be odd to flood the property with a ton of Z4's, considering their new home will be in Germany. Why not talk to the HC staff and the plant to ask if you could continue to use that as a yearly meeting spot. You're promised a good turn out as many have driven to that location before. And there's a reason to have it at that location. I don't believe BMW will turn you down if you agree to do the work in organizing the event and paying for it. Obviously, it wouldn't have the same events but it could be pretty good. Possibly pick up sponsorship from local dealerships or even the aftermarket goods industry.
The VW community puts on some very large shows like Waterfest without the support from VWoA. A majority of their funds come from sponsorship and admission fees. It's a tremendous amount of work. But it can be done if you have some dedicated individuals.