Something else you need to consider is what type of games you want to play. Normal, mislim, kids, ets. Been discussing with a friend for a while on options for a floating milsim game and some scenario ideas. He has some thoughts for rebuild-able structures. Basically your wooden house design, but modular so we could fairly easily change the walls around and keep the assault teams challenged.
As thick canvas will stop a BB so we also thought of basic frames with canvas stretched across. So we could put out cover quickly and move it around the following week or even between games. Paint on sandbags, car, etc, to the canvas front. Just lighter to move and avoids mud build up behind static cover thats used week after week.
I also like to see good safety planning, marshals with radios, designated first aider and in date good quality kit, evac plan, etc. Example of how not to do it: I got burned hands on a pyro a long time back that went off too soon paintballing, I was not impressed to be told they thought there was a first aid kit, somewhere back at the safe zone 1/2 a mile away, could I walk there as they couldn't stop the game. I always carry my own now, suggest if you going to have pyros having bottle of water or waterjel dressings handy along with usual hole patching and some form of splinting stuff if you out in the wilds
Vehicles can add a lot of fun as well, discussed in this month airsoft mag about working ones, but my local site does pretty well with a dead landy strategically placed. If its private land saves on tax/mot, etc as long as you can get it in place somehow.