Hello Commander Ozram!
I think you are perhaps conflating two separate issues: the amount of challenge present in each game mode, and player versus player interactions. I think these are so fundamentally different that comparisons might not be particularly useful.
The challenge of playing in solo being too low (without taking sides) is a valid argument to make, although it might better be phrased as "the opportunities for challenge are too low in Elite Dangerous". It's actually something we are interested in looking at.
However, cranking up difficulty will not make Open more enticing. Conflict between actual people, even within a game, is a very different matter to taking on NPC ships. It has many psychological and social elements that would otherwise not be present. Incidentally, increasing the difficulty of NPC engagements would also make Open harder rather than fairer, so there's also that.
Perhaps the bottom line is the different modes are there to enable Commanders to play how they want to. We don't want everyone to play in Open because we want some sort of Armageddon PvP scenario. We just think that playing with other people, both cooperatively *and* adversarial, can be more fun, which is why we advocate Open play.
So in the context of a karma system, people playing in Private Group or Solo mode are not relevant. Why should folk in Open be interested in what goes on there? This is about making player versus player interactions more equitable *in Open*, getting more folk in there, surely?