maybe a full empire of "fast breeders" should have a war goal of "lebensraum" to make room for their young.
That's not really any different from the hegemonic imperialists, hive minds or fanatical purifiers in fluff, and not different mechanically to anything else. Perhaps a war goal to force open borders & citizenship of your pops so the migratory-minded species can have more options to spread militarily than outright conquest. So in peace they can just migration treaty, in war they can do enforced migration treaties - with the consequence being defeated xenophobic Empires' xenophobic faction having to deal with the sudden forced political integration of numerous xeno citizens, each likely joining a xenophile or individualistic faction.
Loud Whispers:
Reading about your adventures with the locusts makes me think that Stellaris really doesn't do a good job about the whole "numbers and swarm" thing. There's no way to create a Skaven-like or Greenskin threat across the stars as even though your species can reproduce quickly... it doesn't really translate to anything does it?
Yeah the fast breeding trait is pretty meaningless. There's no way to overpopulate your world and create hive cities of megadense urbanization, nor to create dense jungles of sentient plants or chittering seas of chitinous roachoid scientists. Which is a shame really, sorta throws you outta the immersion when arid deserts support the same population abstractions as lush gaia worlds, or when you have in-game fluff of precursor civilizations made up of scattered xenos that live and reproduce over thousands of years.
Hell, I would take numbered stats with icons over the planet tile per pop system. As an aside, when it comes to how buildings represent productivity, I found that modding the buildings to all add adjacency benefits greatly added to immersion and strategy. Early-tier buildings provide flat bonuses to tile production, but as the game progresses things like power plants provide +2 energy, +1 food, +1 minerals and so on to the tiles surrounding them, same for the mineral producing buildings. My reasoning was that this represented how the sum of a planet's parts were worth more than its whole, each tile of nations and cities did not exist as an independent unit sending its resources to a galactic HQ, they were interlinked with one another in chains of production, consumption and supply. Thus with adjacency bonuses, a large planet that is highly divided by hazardous terrain is not as productive as a small planet that is highly-connected, and a large planet that is fully populated and optimally linked provides the full strength of what such a large planet should provide. If hazardous terrain got reworked as a permanent feature and not just a momentary obstacle, this kinda system would even add diversity to every planet ~within~ a biome, before you start adding mechanics to differentiate an arctic glacier world to a deep ocean world.
I mean, sure, you get more people everywhere but once you hit the planet cap... that's basically it. And you don't really build more ships or anything. In essence, it just translates to "we reach the point of peak production faster than others" (though in your case, it's just "lol, we destroy your planets production temporarily"). But in reality, it should be "we must devour the universe because nothing can hold us".
The vanilla game right now has three swarms:
1. Galactic nomad devourers, aka tyranid/zergbugs who intend to devour the galaxy and promptly leave for a new one.
2. Devouring swarms, which intend to eat the galaxy and establish itself as the top of the foodchain.
3. Other hive minds, which want to do what #2 are doing, only more carefully.
All three swarms are hive-minded with a single consciousness. There are no swarms of individuals in vanilla, nor are there swarms that peacefully migrate and takeover planets instead of the usual extermination and expansion campaigns the hive minds conduct. Closest we get to are the migratory flocks, issue being that the nomadic trait and fast breeder traits make migratory flocks marginally faster than other xenos at immigrating and growing within someone else's Empire, forming at best what it says on the tin - migratory flocks like a gaggle of geese, not an epic mass exodus like a plague of chittering (peaceful enough) spacebugs (I also think using the cat portrait would be appropriate, to portray the unstoppable onslaught of useless void felines adopting species as their new owners). Also I really liked the locust pops because they were a type of swarm that was different from the "we must eat the galaxy" swarm.
They make terrible conquerors, what with having no real industry (beyond what is needed to colonize) and no research to speak of. Yet they love making federation friends and making migration treaties, making them into a diplomatic swarm. The damage they cause to Empires is also a form of damage which is unlike any other; where right now your Empire can only be damaged externally by invaders and bombardment, or internally by rebels. The locusts are actually fairly law abiding citizens as long as they're happy, so if they have a planet to themselves and the Empire can afford the loss in minerals/unity/research then they are no issue (unless they spread). That they tank the happiness of other xenos pops makes it spicier, because it means that the Empire in question must contend with internal strife. The planets with locusts and xenos cohabiting form xenophobes, but only on that planet, which means either the rest kowtow to the xenophobes or risk the xenophobes launching a secessionist bid - ironically, with the civ's own species and not the locusts being what tears the space empire apart. Alternatively, the space empire just has its pops migrate off world to core worlds while the locusts take over all the newly emptified space, sucking up delicious materials :>
Basically if the vanilla 3 swarms are hordes of locusts devouring the world's biomass to feed their unstoppable march, the locust pops are by comparison a termite infestation that is eating at the woodwork but really means no harm to the giants is lives with. To that end I think population density and locust pops are diff issues, but by golly, Paradox shouldn't be afraid of fun mechanics
I think this is one of the reasons warfare and diplomacy in general in this game is so underwhelming. Everything is more or less the same with one amusing quirk but still more or less the same. Unless it's expressly scripted like the end-game threats, everything ends up being more or less the same. More or less the same amount of ships. More or less the same amount of damage. More or less the same buildings. More or less the same combat styles.
That and the vanilla game very linearly guides you down the path to exploration, colonization, federation and conquest. There is not even much if any variation in how you execute this linear path. PI taking out trading planets just to stop vanilla locust pop analogues strikes me as bizarre, when the game already runs frightfully close to a cycle of conquer, extinguish and grind, reducing number of fun alternative options to defeat rivals makes no sense. Same goes with having starvation not starve anyone! Madness I say. It's even worse that the trait differences are so small that on the higher difficulties, the weaknesses and strengths of the AI species
do not matter at all. Even on the lower difficulties it's hard to see the variance play out in different ways, once you see one federating species you've seen all of them
Hmm. Maybe fast breeding should make more leader characters spawn. That together with admiral characters who are required to form a fleet, you get a somewhat reasonable military advantage.
Proposal: More pops per tile than 1? Would also allow for xeno minorities, overpopulation and increased growth being actually useful.
Frankly what the game needs is more options than just sending your corvettes to go blow up their corvettes. Synth Empires don't get to infiltrate enemy Empires. Psionics can't literally engage in mind warfare. Gene modding is pretty dank, but the attributes are just too meaningless to alter how a species behaves. Immigration is a means to 10 pop, and never a means to peaceful propagation. You'd think Empires with access to FTL weapons would think of less destructive ways to complete their objectives. The lack of trade sucks. Planetary warfare might as well not exist. Starvation and orbital bombardment doing nothing whilst a measly hydrogen bomb annihilates the planet shows the developers aren't stupid, they just don't want anyone having unauthorised fun xD
/rant aside, a lot of this is rectifiable with modding and Paradox's business model means they'll keep adding features as time goes on. Basically the game'll be good after the first 15 dlc and some nice mods
Paradox instead made a game where everything is just basically clones of each other wearing different skins.
It's a petty complaint I have, but it irks me greatly that hospitals provide bonus pop growth but
cloning vats do not. Thus I always mod them to be a rarer tech that provides a very juicy pop growth