There is another way to create different roles for ships without adding extra military mechanics, namely to diversify the cost. What shiptype is efficient is determined by how much you get per unit of "cost". On paper there are three different metrics for "cost"
- Ships need to be built, a one-time payment of alloys/strategic resources is the "cost" in question
- Ships need to be built in time, an amout of time it takes to build a ship is another metric of "cost"
- Ships have an ongoing cost, each month a ship consumes energy/alloy
Right now all measures of "cost" are almost proportional to each other, battleships are around 4 times as expensive as destroyers in all 3 metrics.
Imagine if instead those metrics were different for all ships. For example battleships had the highest efficiency per unit of upkeep (this is achievable by having battleships have larger fleetpower per unit of navalcap), cruisers had the highest efficiency per unit of upfront cost, corvettes had the larger firepower per buildtime and destroyers were a middle option.
Instantly depending on the situation you would need to build different things. Sudden war you weren't prepared against? Destroyers/Cruisers are your choice (depending if build time or cost is the issue). Got yourself into war of attrition? Probably time to build cruisers and let them die before upkeep bites you. Planning long-term standing fleet? Build Battleships. Feeling adventurous? Let's stockpile alloys and make shipyards with the intention of trying to build enough corvettes on time.
Maybe... I can certainly see where you're coming from in terms of time to produce. Having certain ships as the quick response fleet ("sudden war you weren't prepared against") would be great. I can definitely see that working well.
Beyond that, my concern is that there are a few mechanics in warfare that are based around cost (most notably devastation and armies). The logic is that you will balance military needs against the costs of any given strategy.
The problem with this is that absolutely
nothing is more important than winning a war. No amount of resource costs ever counterbalance winning, nor can any resource savings ever be worth the risk of loss. So I'm not sure that diversifying costs would actually change anything, because costs aren't actually a relevant consideration when it comes to making decisions about a war. You build the strongest fleet you can, and pay whatever it costs, or else you lose.*
For example, if all battleships are the stronger fleet then you have only three options:
- You build all battleships and they don't, and then you win. That's worth any amount of resources to achieve.
- You don't/can't build all battleships and they do, and then they win. It's worth any amount of resources to prevent that.
- You both build all battleships, and then whoever has more wins. It's worth any amount of resources to try and be the navy with more.
That same basic formula would be true no matter what the "stronger fleet" format is.
Resource costs in warfare assume that you're making long-term tradeoffs. But that only works if you have long-term economic concerns that are more important than winning the war. That has never been true in my experience. The stakes of warfare are always more valuable than resources or income. (I would much rather spend every alloy than lose a bunch of systems, no less a planet.) At the same time, there are no consequences to spending as much as you possibly can.
So there's very little incentive to balance costs against power, and absolutely no incentive to voluntarily lose a war you could have afforded to win otherwise. The incentive structure would stay the same: Spend whatever it takes to build the strongest fleet and win the war, then balance your economy afterward.
*Unless you outclass the enemy empire that you can beat them without putting in a real effort. But in that case, you can build whatever you want anyway, so there's no real issue.