I understand your concerns regarding unbalancing the game Paragon. How I had imagined how it could work, it sure would have some "limitations" to sort of balance out it's effects.
- Only one per Galaxy map.
- High cost of resources to keep operational, limiting the empires ability to keep a fleet by 25-30%(maybe more?), regardless of your empire's size. Perhaps resource cost would even be directly linked to rising population on the ringworld.
- Some races might be inclined to want it (higher aggression from some races), while the ringworld would be shunned by other races that would prefer domination through a large fleet.
- Production is limited on the ringworld, after all it's an artificial world and you cant mine it.
- A big tourist attraction!
- Regular disaster's related to the ringworld, a few examples:
--One of our rather ignorant ringworld colonist decides to follow his life long dream of becoming a miner, only to find himself sucked into outer space when he finally gets through the ringworld floor. The ringworld needs fixing before to much atmosphere leaks to outer space.
-- The jet thrusters used for spinning the Ringworld (giving gravity) are failing(lack of resources perhaps?), act soon before the population take an unscheduled flight towards it's sun!
Naturally it's effects would need to be limited, so it might give tax revenue of 5-6 of your average "normal" solar systems, or what do you think?
I think it would actually give an empires a chance to be an economic powerhouse while not "needing" to cover large sections of the galaxy.
You might even toggle your map to be with or without a ringworld at a beginning of the play, maybe even have one abandoned on the map when you start the game.
These are just examples of what has been going on in my head. I guess i'm just trying to say it could have it's expenses and limitations, but could still bring a lot of fun...