Sorry if this is annoying, but figured it couldn't hurt, and at least one of these ideas might actually be useful. Also, yes I am aware just coming up with an idea isn't the same as coding it, but as I am unaware of the difficulties of coding it, I cannot take that into account.
Aquatic and amphibious biters: ones that treat the waters as movable tiles.
Flying: if these aren't the wasps. Basically, able to avoid land mines, walls, etc, but much weaker.
Elemental biters: I'm assuming between fire and inferno, one of them is fire damage and the other is fire resistance (if there is a more detailed breakdown of the biter types, please let me know). But maybe cold biters which slow players, turrets, healing walls, etc. Or electric which could stun turrets, players, and other machines (long cooldown so it doesn't just stun-lock all your defenses).
Hydra biters: they have a regeneration factor, and after healing a certain amount of damage, they split into two biters.
Pukers: spitters with a short range AOE cone.
Healers: heals and buffs other biters in an area, but the inverse when they die (damaging and briefly debuffing them).
Cannibals: when biters die around them, they can stop briefly to "eat them" and not only heal, but grow stronger than before.
Plague biters: infects structures in a way that slowly damages them, and turns them to the biter side if they are destroyed while infected. Repair packs can counter this. Or fish/med kits for the player.
Artillery spitters: spitters with the range of worms.
Chain Spitters: rapid fires small shorts.
Creaper: spreads biter creep, which increase biter speed and decrease the speed of everything else.
Harvesters: collects pollution like a hive wherever they are, and adds it to near by hives.
Web worms: basically make walls and gates for biter nests. Those things go away when the Web worms die.
Carrier biters: holds weaker biters internally and disgorges them near (spitters)/at (biters) the target. Protects them from damage while inside.
Launchers: yeets biters to the target, over obstacles.
Mortar spitters: hurls explosive globs.
Kaiju: a special 100% evolution type that are really slow, but really powerful, coming in a variety of type. Basically boss monsters that can threaten even end game bases without either layers of defense or a lot of infinite research.
Space Biters: something for Space Exploration, ones that can attack space ships and space bases, and deposit new biters on a planet orbital drop style (potentially even into the base if unlucky). Nests can build more of these space biters like kaiju. Meteor defenses can intercept them, but the planet covering ones stop only half, and it can overwhelm one or two normal ones.
Those last two are more just potential ways to keep them relevant when biters are normally no longer an issue.
Anyway, hope any of this is a good/useful idea, and sorry if not.
