OK, I follow Classic Traveller rules for the most part, but the biggest change I have made is the use of antimatter. I make the cost of antimatter at 5x refined fuel, so any ship that needs to run on it is EXPENSIVE to operate. The positive side is that antimatter containment vessels can hold ten times the equivalent energy, so a jump 1 only needs 1% of the volume of a ship.
I have my express couriers run on antimatter, and that eliminates the need for express courier stations every four parsecs, as an express courier can now go twenty parsecs without refueling, but most of the time they go for four jumps, where I have the next express courier station. This means that express courier stations need an antimatter processing plant, and those need to go someplace far away, like an uninhabited planetoid circling a gas giant.
The Imperial Dreadnought Fleet also runs on antimatter, which allows it to respond very quickly to emergencies, and for individual ships to jump into a system, and then jump out again without refueling. Running these ships is EXTEREMELY EXPENSIVE, so the Imperial Dreadnought Fleet only gets activate if the problem cannot be solved at the Sector Navy level.
For the sake of a story I have written I created a plot device called a Maker. It is a molecular assembler that can build small electronic, electogravitic or electromechanical devices using an ion beam to position individual atoms. It works best at the nano-scale, but it you have a lot of energy and time, it might be possible to assemble a spaceship one atom at a time.