Very interesting insights, very much appreciated.
Preliminary Recommendations for setting up NPRs:
Do not allow NPRs to appear randomly; this will dramatically slow down the game, especially over time. Do not automatically create NPRs at game start unless you are prepared for a game of *very* random difficulty - either terribly difficult or boringly easy, and you won't know which for many hours.
So, don't play with NPRs?
I agree that NPR econ growth is always inferior to that of an empire run by a remotely competent player.
This, as you say, is why NPRs created at game start will only ever be a challenge early on.
But NPRs that are generated randomly during a game are a different story.
These NPRs have an economic strength at creation scaled to the player's economic strength at that time.
(I believe that the exact scaling factor used for a given race is selected from a random range centered on the player's economic strength, multiplied by the game's difficulty factor.)
In other words, you can't out-econ one of these NPRs until after you discover it.
Furthermore, the new NPR creates its entire fleet at that time, from whole cloth, using all of the available shipbuilding points.
This is quite an advantage, because the NPR gets the benefit of a completely up-to-date fleet.
By contrast, the player does not know exactly when contact with another race will occur, and therefore will not have an ideal, up-to-date fleet at such time.
Therefore each new NPR generated will represent, at the time of discovery, an immediate challenge to the player, having an economic strength scaled to the player's own and a fleet that is likely much stronger than the player's.
The best way to mitigate the challenge is to explore fast and far.
You want to discover these NPRs as soon as possible, as far away from your economic assets as possible.
You want to discover them far away so that you have as much time as possible to make up the fleet readiness gap before a significant engagement occurs.
You want to discover them early to minimize their starting econ strength. The longer it takes to discover them, the stronger your econ becomes, the greater their starting strength.
For me, these factors lead to a satisfying play experience.
I methodically develop my economy, slowly integrating and exploiting additional systems further and further from my HW, careful not to commit too strongly to a position until I am capable of projecting significant power there.
Meanwhile, I push the exploration frontier out as fast as I can, creating minor colonies for supply and maintenance along the way.
I'm 37 years into my current game (15% research speed, 10% survey speed).
I have discovered 50 systems at a distance up to 9hops and 29.4bkm from home.
I have a total population of 1.4b, but less than 20m are more than 3 hops / 10bkm from home.
When an NPR is discovered, I am at an immediate disadvantage in the area near the NPR.
The theoretical midpoint of equal effective power projection between us is probably closer to my HW than to theirs.
Overcoming this disadvantage takes time, strategic planning, and effort.
Which is kind of what I like from a game.