I was thinking about how arbitrary the whole "eligible additional classes" and refit cost is and how unsatisfying designing a ship actually is.
It hit me that rather than designing ships we should be designing ship hulls. Ship hulls that we define to have a certain size, engine mounts of certain sizes, number of weapon mounts of various given sizes, internal system spaces of given sizes, external system mounts of given sizes and so on and so forth. Of course, what is realistic and appropriate to class together to allow to be fitted or mounted in a given space of mount would have be carefully considered.
Anyway, then that hull would have to be researched as any other race specific tech and then a shipyard tooled for said hull. When that is done ship variants can be designed from said hull (take inspiration from Star Citizen and how they design ship variants of ships with identical hulls). For example a large pressurized internally empty hull section could be used for hangar space, cargo, cryogenic transport, troop transport and so on depending on what components are installed in that section. Or a ship could be outfitted with a more powerful but fuel consuming engine of a size same size size to create a racer variant of the standard layout. The shipyard would be able to build it all the same as all they do is build the hull and fit the components and leave the construction of said components to planet based factories.
This means that older ships can much more easily be refitted with new components, meaning that you don't have to throw out an entire design when you make some minor improvements to some system. You just replace the engine or whatever. Its also more realistic as fx modern submarine design is all about modular design as hull design is often a major cost of development. The only time you should design new hulls is when there is a shift in fleet doctrine or trough some paradigme shifts in technology that necessitates entirely new hulls (maybe some new line of research that affects hull design?).
One could consider partially keeping the old system for cases where you want to design a ship from a hull with systems different from what the mounts and hull spaces allows. Say and extra engine to make a speeder version. This could be allowed but sees a logarithmic increase in cost depending how much the ship differs so to allow a one to create rare and expensive "elite" ships for cases where you would normally design a new hul but you want just a few ships of that variation so you modify an already existing hull and produce it at the shipyard for that hull, at a much higher cost than normal ship variations.
Finally, one could then also consider making possible to design sections that can be produced separately and later be put together later. Like engine section, mid section, front section or what have you. Especially useful for capital ships, modular stations and creating unique ships fitted together from various hulls sections (Then we can have cool randomly generated pirate ships and stations slapped together from various hulls of ships they have captured all over the galaxy, think of the possibilities!
) At some cost the ship health or production cost whatever seems realistic and balanced.
Anyway, its just what I have been mulling over it and this is just my early thoughts that I want to throw out there to hear what people think, the particulars can be changed later, just consider the basic concept of what I am suggesting.
I know there is a suggestion thread, but I feel this would be a major change and generate a lot of debate, so I made a new thread. Sorry if I broke any rules.
Edit: It may also be worthwhile to consider this for Aurora C# given the how much it would likely change the game.