I understand that the devs would be hesitant to ad actual living enemies, as that would raise their ersb rating.
I think that Aggressive Magic Rocks would be a better alternative to an enemy nation
It would make sense for them to be scattered around the Kerbin system
You aren't killing actual sentient beings
You could collect fragments for science/funds
You would be able to keep the E rating, and not have to feel bad about killing anyone
They would be made of fewer parts and not tax the performance as much
They could have their difficulty tweaked/ be removed if you don't want them
You could have enemy vessels composed of different parts made of "magic rock"
"beamers "that shoots lasers a relatively short distance (500m or so)
"flingers" that detach and launch themselves to nearby craft (2km)
"jerkers" that fling anything that gets near (100m)
And of course you have larger rocks that these weapons are attached to
Big glowy rock - provides power to other rocks attached to it, is large and has a lot of health
floaters- provides a force that keeps the rock suspended about 10m off the ground
Seekers- provides a force that moves the rock towards nearby craft (6km)
Flyers, wing like rocks that somehow propel themselves through atmosphere
I know this is a lot, but this idea just came to me and I had to let it out.