It's a common theme in science fiction for a starfaring organisation to have a single starship of enormous power. Examples include the Superdimensional Fortress Macross, either of the Death Stars, the Homeworld Mothership, the Ur-Quan Sa-Matra, the Voth city-ship, etc.
I'd like to suggest that a new tech - or series of techs - be added to the end of the hull tree, allowing a new, absurdly large hull-type to be made available. This hull would be limited to a single ship per empire. In this way, any empire dedicated to the path of hull-fu would gain access to a one-off ship to be used as a centerpiece, sledgehammer and ultimate expression of military power.
Such a ship would be many things. It'd be like the wonders that each empire can build once, it would be the 'game winning' tech for conquerors, and it would be the coolest damn thing ever. What does everyone think?