A couple of observations...
Firstly, do you know that FD don't already do what you are proposing, to a level that they feel is fair or correct? Other than random NPC traffic and interdictions, any time you take a mission, (and therefore presumably in PP and CG's too), the game generates NPC content specifically for you.
Secondly, QziQza does have a point, that what may seem like a small change can have more wide ranging consequences. To use your example of 12 Anaconda players trying to blockade a station, so putting a similar number of equivalent NPCs into everyone's instance is actually very unbalanced, as presumably only a percentage of Open players would be instanced with the player Anacondas, yet every Solo player would have them as opposition, making Solo very much more challenging than Open, much less fair, to use the Open only people's argument back at them.