I would keep things pretty much as they are, but I would try the following:
Increase the LeMans grid size to say 60
create a category for cars from the Japanese GT series, also allow TransAm cars back in.
Create an invitational class,for privateers, at the organisers' discretion, for vehicles built to past rules that may still prove popular with the fans- GT1 cars like the Toyota, Porsche and Mercedes. This would encourage privateers to enter cars that the factories could no longer use.
Allow automatic entrance to qualifying for any car/driver combo that has won another GT series/Sportracing series outright.
Have an invitational category that changes each year for vehicles like Sports2000 or Australian V8 supercars, to increase the sppectacle and create opportunities for fans of existing series to get involved.
Have a prequalifying race for Porsche GT3Rs to cull their numbers a little
