Yes, I think many people are reading this thread and assuming by "supercapital" we mean "build this and win", which is not what I was suggesting. A SC would be less powerful than a starbase, by quite a way, but we all know how easy it is to take down a fully upgraded SB with a half decent fleet.
All I'm asking for is something unique, that is clearly leading your forces. It doesn't have to be that powerful, so long as it is stronger than a standard capital. Due to the way the game works it would be easy to stop players "just getting a supercapital as fast as possible then winning" - just make it so you can only build one, it's perhaps 2x more powerful than a standard capital and takes 3-4x the resources and supply slots. This way, it would be easy to balance - you could even make building a SC a disadvantage in terms of sheer strength, but about even if you consider the extra abilities a supercapital would have.
In terms of gameplay, it would be cool to see your enemies fleet approaching and know that because their supercapital is included, this must be their strongest fleet, or maybe everything they have (because why would you risk your ONLY supercapital?) This works best with the "make a supercapital from an existing lvl10 capital" idea - because once you lose it, it takes ages to get another, rather than just being able to churn them out every time one gets destroyed.
Of course, this opens the door for plenty of cunning tactics - like I said, some sort of bonus for taking out an enemy SC would mean you have to protect it, and clever players could intentionally risk their SC to force their opponent to do something they want - ie. send a diversion fleet including your SC, which the other player assumes must be important and rushes to counter... while you sneak round the back with your main force.
Again I say, I'm not asking for a ship that can take on a whole fleet and win. I'm asking for a ship that's like a capital ship (weaker than having smaller ships in terms of bang for buck, but has useful support abilities) but stronger than current capitals, and unique. Capitals are cool, don't get me wrong, but when you have two of each in every fleet, and the next player has the same... well they just become like any other ship.
It's like a food chain, one of those pyramids they used to teach in biology class - the capitals are not the top level, there are simply too many of them: the current "pyramid of power" in sins has a flat top. I'm asking for the peak! I'm not asking for unbalanced (rubbish) gameplay, which is why I doubt I can find what I want in a mod (never played a mod for any strategy/rpg that's as well balanced as the vanilla game IMHO).
I don't know if the others here who said "yay" for supercapitals are looking for the same thing, but this is my view anyway.