Some mutation trees deserve some tweaks and/or new added mutations.
-The armor mutations (I’m looking at you, Chitinous Plate) need more armor value. I think that Chitinous Plate should have Chitinous Armor Set’s relative values, with some bonuses. It should feel like natural armor, not like a mutation you need purifier to get rid of.
-Troglobite needs Good Hearing and an upgraded version, Perfect Hearing (for post thresh) which gives the pin-point location of sounds.
-Insect desperately needs two upgrades to Proboscis (quit Insect tree after finding out that Prob lets you eat only from flowers): a Bloodsucker tree, where you get nutrition and HP from attacking enemies with your mosquito-like proboscis; and a Precise Proboscis tree, which increases the time you need to eat stuff, but you extract the necessary nutrients from food, giving more nutrition and letting you eat normal food.
-A mutation which “shrinks” Post-Thresh Insect would be good. +2 DEX, -2STR, and +1 natural dodge. An upgrade would be an active ability, which would be an analog to Uncanny Dodge. If flies are hard to swat, so are you!
-A Burrowing mutation for melee-mutants would help them to close-in on ranged opposition. It is an active ability. When you burrow, you move underground, create dirt mound tiles (like a worm), but cannot move under concrete and other hard stuff (an upgrade would eliminate this). You get spotted when you are 1-tile away from your enemy. You cannot be shot while underground. It’d work for Chimera and Trog for thematics.
-An Acid-Spitting mutation would work for Insect. You spit acid like a… spitter.