While this is quite true I do not think there needs to be a lot of innovation or new enemies to add to this state of the game. Right now there are individual descriptions for each mutation "stating" that certain arthrons possess different capabilities with the later ones supposedly taking a "commanding role" over other versions of its species. This specifically could be implemented as a couple of new heads/arms/carapaces to reflect that description, and as they have taken the time to write these this was probably something they wanted to implement.

Something I could see as a great improvement would be to tie the umbras to specific melee classes of arthrons that ruch in to try and "suicide" as it itself knows about its post mortem capabilities. And tying buffing or lesser psychic capabilities to supportive arthrons with viral machine gun would also make a lot of sense!

I have only played the one year edition so I don't know about how it has been before, Hopefully i'm not suggesting old stuff!

The larva concept art looks nice https://phoenixpoint.info/blog/2017/10/29/new-jericho-starts-to-take-shape

Make one new line of hard -type soldier at human foe/friendly fractions bases-something like leader of base defense forces,like coordinator, with new support /attack skills,like military-named mini-boss.Can random release in defense base forces,when we attack/steal on enemy base

A sea snake. Spits poison and paralyses.

Trilobite which is hard to shoot due to very low height. Envelopes the soldier and kills it the next turn if not interrupted.

Squid-like creature that grapples soldiers entangling them and slowly drains hp from them. Can grapple several units at ones. Shooting tentacles releases the grapple.

Leech: Similar to mindfragger but instead of mind control do high damage every turn when attached to soldier. Can by removed same way as mindfrager.

Acid and poison blood mutation. When damaged releases acid or poison in small area.

Mist worm releasing mist cloud and mist worm chiron.

Grappling tentacle. Arm or torso mutation that gives unit tentacle capable of grap and move soldier next to attacker.