I wrote about this in my original post. Here are some suggestions about implementing it given the absurd configuration of MLB ball parks.
1. Decide on a distance that a fly ball should travel to be a home run, let' say 380 feet. Measure that distance from home plate in each park, into the stands where necessary. Mark the 380 foot line in the stands. Any ball that lands in the stands but not over the 380 foot line is a ground rule double. Move the fences in at any point where they are more than 380 feet from home plate.
2. A fly ball must be caught in fair territory to be an out. This addresses the disparity of the area of foul territory.
3. Determine the minimum foul territory and mark a boundary in all others to match that area. Any ball that goes over the boundary is considered to be in the stands.
4. Make the height off the fences the same in all directions in all parks to the extent possible. Ten feet seems about right, preventing players from jumping into the stands.