On topic, but also off it a bit, I'd like to see a couple of smaller (say 3 or 4 star) independent systems. Something like the United Baronies of 'X' that is a four star unit that is independent but something like vassal state of the Empire. Or the five star system free trade block of 'Y' that the major players use as a banking and diplomatic neutral ground.
Think places like Switzerland or Luxembourg. Think places like Cuba or North Korea. Think Armenia between Rome and Parthia. Buffer States that live by their own rules but are either subservient to a greater power, too important for either power to let the other have, fill an economic or political need that means all powers are happy to let them live unmolested, or that they are too unimportant and / or would be a drain on the treasury of the big players for them to waste their time and blood annexing them.
I realise I'm probably too late to the party to see this suggestion see the light of day in the game that is probably almost set in stone, but I think 3 big players, 15 or so minor players and the occasional single star system that is allowed to survive because the vagaries and chance of geo politics make them more useful as they are then any other outcome would add some wonderful colour to the game.