I was thinking one thing that is already in game. Barbed wire!
Make barbed wire so it stops foot traffic and is attachable to the top of base walls and gates. Barbed wire would also have to be destructible or every building would be blocked off in a day or two.
In addition Barbed wire should also either only slow or cause very minor damage to vehicles. Personally I lean more towards slowing vehicles with no damage or the roads will be nothing but barbed wire as far as we can see. MAYBE also add in that the vehicle destroys the barbed wire.
In addition to the barbed wire change maybe double or even triple the health of bases. I'm not sure how easy or difficult they are to knock down but h