OK, I'll stick my neck out with some suggestions.
Reduced aero grip - single element front and rear wings with reduced size end plates. No aerodynamic parts between the axles, ie no flip ups, barge boards, horns, third wings or anything else of that nature - I was thinking of a rule which stipulates minimum width to thickness ratios for bodywork to enforce this one.
Underbody aerodynamics - no skirts but venturi tunnels of a specified size to offset some of the aero losses. This should allow cars to run closer.
Increased mechanical grip - big fat slicks, tyre changes allowed but discouraged, see below.
Reduced engine cost - 4litre normally aspirated V12s, no pneumatic valves, mechnical valve springs only. By reducing engine cost the daft 2 race rule can be removed. Valve springs mean a rev limit of about 12k rpm so no need for expensive exotic materials. Onboard starters. Capacity increase to offset power reduction, V12s because they sound better than anything else
Refuelling - banned, fuel tank size to be in excess of what should be required to complete the distance, therefore no need for fuel conservation tactics.
Pitstops - the idea here is to allow stops for tyres but prevent the races reverting to 4 sprints on fresh tyres. So Le Mans rules apply - no more than 4 people in the pit box at any one time (driver not included). No pneumatic jacks in the cars so realistically one guy does each side of the car. If you want more than 4 guys working on it at once it goes into the garage, engines must be stopped before pushing into the garage and may only be started on the pit apron.
Practice & Qualifying - no lap limits in Friday or Saturday practice. Qualifying, 1 hour 12 laps is the best format I've seen for entertainment. I'm not sure about tyres though, 4 sets of new tyres is expensive but by far the best option for a spectacle, so what the hey, let's have qually rubber back!
Driver aids etc - manual gearboxes with mechanical linkages, manual foot operated clutch. No traction or launch control, I don't like the idea of standard ECUs but if that's the only way to police these then so be it. No ABS, no anti stall (no big deal with on board starters).
Standard components - no defined standard components but remove restriction forcing teams to build the entire car themselves thus allowing for purchase of whole or part cars from other organisations.
EDIT: Forgot a bit in the aero regs...
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