You have some really good ideas there and I could not help but respond. (Love 4Xs and Moo2 and I am fascinated by design as I have done it for boring business applications).
Multiplayer would be nice but not a deal breaker if not present
Moons, yes! let there be a role for moons in the game, it would certainly add to the immersion and realism
Planets your suggestion on Ringworlds and dyson spheres being formed would be a personal put-off in my case. I don't care for what this would do to the game. Specifically, recall that in Moo2 an invasion fleet had the option to attack any single planet in a system or just park and blockade. This lent many choices to the player that are lacking in the 4x games made after Moo2. By creating a ringworld or dyson sphere you remove my ability to degrade a solar system in the manner of my choosing and limit my tactical options. Just my opinion, but I would not care for this. I also never understood the complaints about micro-management in Moo2. By late game, most of my planets were producing trade goods (no management at all) and my few industrial worlds were churning out ships. By renaming systems and using the same names game in game out it was easy for me to identify my industrial worlds and micro-managing was never an issue.
Leaders do need to be enhanced, I suggested that they come with unique researchable techs as well as blueprints for normal tech. Your suggestion would be a nice bonus.
Technology Yes balance is crucial!!!!!!
Star map / ship ranges 100% totally agree!
Planet/system development your suggestion is very intriguing, perhaps development should also be tied into total planetary population otherwise it would seem a bit awkward for a newly established colony to be fully exploited within one turn. Perhaps if you also were required to transport additional population along with the structures it would be effective.