So you do the exact same thing but with money?
Well maybe not the exact same thing as I am more of a technology optimist.
What I am suggesting is a system where the production levels of your industry would rise and the production costs would drop when the technology develops. Now with this system the production levels of the industry which is maintaining your fleet would of course rise too and as a result of this you would probably need to have only little more industrial capacity to maintain the same number of bigger ships than you needed to maintain the smaller ones.
So I would have the production times and costs of your bigger ships drop, like in MOO 2, when the industry develops even though the ships would naturally have more advanced technology in them, as I believe that the industrial development would keep up and maybe even a little ahead of the technological development of your ships.
I would also allow the number of ships that you can build to increase atleast a little over time even though the ships would get bigger at the same time. This would show the player the clear effect of technological development.
Like I said earlier I would limit the number of ships by using maintenance costs. These could include things like the crew costs (salaries, food etc.), repair costs and the over all maintenance of the ship, like changing old parts to newer ones. Now these things cost money and the bigger the ship the more money its maintenance would cost. Of course when the technology develops these costs should probably drop as well and so you would be able to build a little more ships than before. But if you would build too many ships your economy would be in ruins as the up keep of your oversized fleets would require almost all the money of your empire. And so you couldn`t develop the other areas of your empire, like planetary infrastructure, technology, espionage etc. which would also require money.
The probelm is that the Size of your fleet can only be determined by one of two things
1. Empire Ship Production (ESP) / System Cost
2. Empire Maintenance Production (EMP) / System Maintenance Cost
Now ESP and EMP would both be
1. Empire size * Tech Factor (where they may have a different or similar tech Factor)
System Cost and System Maintenance Cost, are both a Pure Tech Factor
So the Size of your Fleet will always be
Empire Size * Tech Ratio
(where the tech ratio is one Factor/the other) depending on what the controlling Factor is.
I personally think that that Ratio should be nearly constant, every time you get an industrial tech it goes up, every time you get a new weapon/ship system it goes down.
Now for having the ratio be drastically different between Production and Maintenance, also creates a change in the game.
Basically, Production Cost/Maintenance Cost is a Time Factor... How long it would take you to fully replace a fleet without changing your overall level of military ..investment. Ideally this should Not change drastically with tech level, because it changes the nature of the turns in the game.
The reason to keep that initial
Fleet Size=Empire Size*Tech Ratio
the same is to prevent
1. Early battles being boring because of too few ships
2. Late battles from being boring because of too many ships
You would still get more ships later on, but that is because you are colonizing more planets, getting those planets fully developed, etc. New technology should Not be a key part of it, allowing the pace of the game to be maintained throughout
For more control, each goal could be given a number of "points". The AI would then decide how to pursue those goals in order to maximize the number of points it earns.
I think something like this is mandatory, a simple this first, then that doesn't work (especially with self preservation.... because that is much more like will you risk 5%, 20% 60% of the fleet to achieve this goal... ie what are the levels of acceptable losses)