You mean you want to be able to blow some one of your own side out of the sky with out retribution?
Get real.
That's not going to happen, and rightly. Look where you point your guns, it's you pulling the trigger.
On the other hand, that's not to say the penaly it's self can't be tweaked. In AW we had "PNG" (persona non grata); kill a friendly and you had no ammo for a day, and were thus forced into eother flying for another side, or flying goonoes. Personaly, compared to that, I prefer kill shoter.
What have other sims had? Propose a workable alternative.