The game that comes really close to this method is Starships Unlimited. I love that game. The ships are fully customisable and you can rarely maintain more than 15-20 ships, as they just cost too much upkeep, plus every time you buy one it costs more than the previous one.

The main problem with this method is that you can’t really defend your systems as your empire gets bigger. If you have 50 systems but only 20 ships then it’s difficult to always have them at hand when needed.

The last thing I want to have to do is go into the “education” screen for each of my 30 planets and allocate spending to train the next generation of cooks so that I can outfit a fleet with chefs 10 years from now, while bartering with the planetary trades union and constantly monitoring their contractors to make sure they are getting the job done on schedule.

There is a great possibility for depth in ship design and creation, but the question is all in scope. A typical 4x game has to abstract a great deal out, or else the whole thing becomes a giant nitpicky micromanagement festival.

It’s better in my opinion than five, also easily moddable with tons of mods

Yes it has it’s flaws but hey so does every game
The best thing about it is it’s huge community and Mod Librarys

It depends on what the game is going to be,

If it is an Empire builder, then look at my first post… keep ship production simple, and keep ‘fleet size’ technology independent by increasing expense.

If it is a ’space ship role playing’ or tactical game. Then the planets should be more generic, and you treat your fleet like a group of Role Playing characters…. the process should still be Simple… but instead of building+designing mass producable ships, you would focus more on individual ship parts/crew
also
Tasks that you do with your ships (capture planet X their scientists know how to make Hyperdrives) to get ship parts or better crew (carry the precious carge to Empire Y so that one of their navigators will join your crew)… the planets and what you do with them is secondary.

Ship construction+maintenance should be Simple, expensive maybe, but Simple.

1) The difficulty of UI dealing with Many ships (in MOO1 it was fine, you could just as easily make+operate hundreds of small ships as few large ships) MOO2 had problems with that. Eliminate, those and many, small ships gets more fun.

2) The economics of technology levels. this will take some terms being defined

Total Fleet Space: presuming you have “space” on ships that differentiates large ships from small ships, this is the sum of all the space on all the ships in an empire’s fleet.

Price/space: With any particular set of technologies (weapons, drives, etc.) available, the amount of production or money it takes to build a militarily useful ship divided by the total space that ship provides.

Output/planet: With any particular set of technologies (factories, popboosters, etc.) available, the amount of production or money that a developed group of standard planets can produce divided by the number of said planets.

So
At a Particular Tech Level (this is the Important term)

Maximum Total Fleet size= Number of Planets * (Output/planet) / (Price/space)

Fleet size normally spins out of control because
(Output/planet) increases significantly
(Price/space) increases little if at all

For most games
at “Technology level 1″ a single developed planet can to build a fully equpped small ship in 10 turns
at “Technology level 10″ it can build a massive spaceship in 10 turns
Because the planet is producing 10-100x as much and the Massive ship costs 10-100x as much as a small one. The newest High tech parts cost as much as low tech parts did when they were first discovered.

The ideal would be
at “Technology level 1″ a single developed planet can to build a fully equipped Medium ship in 10 turns
at “Technology level 10″ it still takes 10 turns to build a fully equiped Medium ship.
Because even though the planet is producing 10-100x times as much…. the ship costs 10-100x as much because it has more expensive (and better) parts on it.

Now you need to make sure that the more expensive high tech parts are a better option almost all of the time than lots of cheap low tech parts, but that just means adjusting damage and hp both up, or armor+penetration, or range+speed or sensors+stealth.

In this method, your Fleet Size becomes a representation of your empire size. So a 100 planet empire could have
thousands of small ships, hundreds of medium ships, dozens of large ships, or Several huge ships [assuming it was very militaristic] Those numbers would be Independent of where you were in the game.

3) The economics of maintenance
This is the ‘how my fleet size is determined’ is it by how fast I can produce ships, or how many I can maintain.
Now in a serious war, how fast I produce them is almost always more important because I am losing them.
But in “peacetime” there should be a balance where maintenance is an important issue too.

If ships are expensive to maintain (campared to how much they cost to build), then a fleet just won’t just keep growing through times when you aren’t fighting serious battles. (Of course they shouldn’t be too expensive, because then your ‘whole fleet’ is easy to replace… because the cost was mostly maintenance, not production)

I’d say basically, you should be able to ‘reach maximum fleet capacity’ from 0 in 15-30 turns (depending on how fast the ‘civilian’ economy develops and how fast ships actually move across a typical empire)