TL;DR
We need more kinds of enemies that behave and do things differently to create new and varied problems to solve well into the late game.I'm aware there are plenty of posts about "New/More/Type XYZ enemies", but those seem mostly about "this would be neat" or "I want" more than how that will help drive the core purpose of the game. I'm more focused on the justification for the concept as a whole rather than requesting any specific kind of enemy dynamic.
What ?
There are many idea out there already, the point here is to demonstrate why more varied enemies can drive factory design. So understanding it ultimately doesn't matter what the variation is so long as it creates a corresponding factory design choice, here is a brief-ish lexicon to reference;Flying Scout (why shouldn't the biters be looking for things?) Patrols around bases or just goes looking for players other spawner bases to attack.
Flying Biter - It can fly, but lands to bite.
Flying bomber spitter - Heavy attack "long range" (Either high altitude limiting counter attack options or literal long range)
Flying Spitter - light attacks short range attack.
Digger Biter/Spitter - (Attacks as normal once surfaced, Think of a sand mole, doesn't really leave a traversable tunnel in it's wake)
Digger Tunneller - (Same as sapper, but does not have an attack, instead provides an access route for additional biter hordes)
Artillery spitter - Moves slowly, no melee or short range attack, but long range equivalent to auto range of un-upgraded player Artillery.
Flaming spitter - Essentially a flame thrower unit.
Kamikaze biter - Blows up, particularly useful for clearing mines ahead of a horde due to AOE.
EMP Biter - Stretching it here, but if we have electric eels it could be reasonable to think an EMP might be possible. In any case, disables electrical devices temporarily. Aka Lasers and inserters. Leaving flamer throwers "unaffected" I know they are automated but I figure a pilot light and back pressure in the pipes gets you most of the way there.
Jammer Biter - Disrupts radar preventing remote viewing (detailed zoom) and updates of the map.
That should be sufficient to start with, but one more possibly unique suggestion amidst this is the idea of speciation. Aka different species of biters that bring different subsets of the above units to the field as well as different species traits like having Armored or "Silver" (Like the silver ant) carapaces making them highly resistant to bullets/explosions and lasers/flames respectively.
Why ?
I'm not interested in creating an RTS out of Factorio. Pretty quickly, enemies transition from being a real threat to a mere nuisance. I work toward building a wall, lined with gun and flame turrets around my entire base and after that, we're pretty much done. I don't even bother with laser turrets. Between bots repairing walls, dropping wall barricades out side the main wall to slow down the biters, and flames sitting underneath it all things die pretty much instantly. Problem solved. Permanently.Which is no wonder, at present we have two verbs. Melee and ranged attack. Delivered exclusively by running either at the nearest aggressor or source of pollution. If we add a few more, the problem gets more interesting. lets take tunneling to start with.
If a Digger sapper can show up at some stage of evolution, you need to account for it at some point. Traveling under ground would make it invisible on radar, so you'd have to count on visually sighting it. Brick and concrete might slow down how quickly it can surface underneath your base, and refined concrete might block it entirely. In the absence of concrete or brick you need to pepper turrets through out your base. And you can't afford to use flamers as they are indiscriminate.
Later on, Digger tunnellers become a problem. All of the above considerations apply except that as a bigger more powerful digger they can punch through refined concrete. If you can kill it before it retreats it plugs the hole temporarily until the units behind it can carve their way through. Then the Kamikaze units come through first to blow up the perimeter and provide a landing zone, followed by standard biters. As explosive units, lasers will detonate the kamikazes, but not gunner turrets because they basically just use lead or uranium slugs. Non-incendiary. And now there is a permanent route underneath your defenses into your base. The tunnel path is visible from the surface, and you can collapse it with artillery rounds or rockets to the surface. The tunnel decays over time if unused, so by destroying the entrance you effectively destroy the entire tunnel unless a new tunnel happens to intersect the old one before it collapses fully.
Here the Radar jammers come into play. If your radar can't see the entrance you can only collapse it as far as you can see, right up to the edge of the radar jammer. Which would preclude "scouting" with Artillery shells. And if Radar jammers accompany the units in the tunnel, you might not be able to see the battle when they emerge from the tunnel in your base.
Additionally, the tunnel could have material benefits to the biters. the excavated material has to go somewhere so maybe it shows up as a stone field you can mine or just boulders dispersed defensively around the entrance or even both. Stone becomes a resource the biters use and deposit as a new stone field when they've built all the defenses they needed. Either around the tunnel entrance (which doesn't have to be in their base) or around their whole base making destroying a base more difficult.
Flamer turrets could be useless in dealing with flying or tunneling units, Gun turrets and laser turrets are accurate hitting flyers and gun turrets less so but more damaging, Lasers cause kamikaze units to explode but not gun turrets, which might be leveraged to preserve a mine field inside the range of a gun turret. Or alternately laser turrets to hit flyers while forgoing a mine field.
So this handful of dynamics, is responsible for pushing the player to build more varied defenses specific to the threat presented, and to consider prioritizing paving their base (which I usually ignore as a luxury rather than a need), pushing towards having rockets prepared or pushing eventually to artillery (I don't usually carry a rocket launcher since I'm always in a vehicle or behind my walls)
Getting speciation into the mix where regions are dominated by different species (One with stander biters, flyers, and emp biters vs one with standard biters, tunnellers, and radar jammers for example) that war with each other as well as the player means over time your strategy for dealing with one no longer deals with the other requiring a redesign of your defenses. Artillery can keep species with no tunneling at bay but the tunnellers can slide right up to artillery and destroy it for example. And if the local species is resistant to one damage type or the other your defense must be aligned accordingly. Likewise making forays into the unknown becomes more strategic depending on what vehicle and weapons you sally forth with.
I do think this would require a few more turret/ammunition types. One or two more remotely controlled but less powerful units than the Spidertron would also help here. Particularly if you can setup a patrol path for a unit. Then you'd need a factory to produce those units and all their required resources. But that would be a whole other suggestion.
In any case, that's more than enough to think about. I'm sure there are more interesting ways for the natives to drive design decisions while at the same time keeping things interesting from the beginning through and past the late game. At present, once I've got a gun turret and walls, enemies mostly become tedious.
But the point is: adaption is not variety ; you don’t need to add more types of enemies to have a better adaptation.