Horse line reminds me of an idea I had a while back, that seemed odd it wasn’t a thing; “Ungulate” line instead of “Cattle” with post-threshold specification. That way you could get things like antlers (“Now I am the moose!”), a goat’s iron stomach, or a gazelle’s speed. That’s probably me over complicating things, though.
As a counterpart to the Alpha mutagens, what about an Omega set? That is, one that focused on transcending the form, rather than amplifying it. Mutations could include things like a third eye, “life sense” (basically infrared vision), maybe even ominous hovering or telepathic projection post-threshold? (The projection would work like Growling Voice, but with a cooler descriptor.) Have the negative mutations be things like frailty, glowing eyes (try sneaking with those on), blue/grey skin that NPCs find off-putting. I don’t know. Not sure it fits with Cataclysm at all, just a thought I had.
Bioluminescence: You’ve effectively got a permanent glow stick… that you can’t turn off. Would work for the fish or insect lines. Maybe could progress into one with an on-off switch?
Hive Mind: Being around insects gives a stacking mood debuff, as your character deals with excessive mental “noise.” Alternatively, if you were around enough of them (hive or hill), it might do the noise effect that schizophrenia sometimes gives.
If an amphibian line ever goes in, one could add a wood frog’s cryoprotectants to help compensate for low temperatures. (They straight-up freeze while hibernating, thaw when done.) Or, for a theoretically sweet mutation, a hairy frog’s bone claws. They literally break collagen bonds and force sharp bones out through the skin of their digits to defend themselves. Would be basically activatable claws, maybe with some damage from activation.
A Worm line could be pretty cool. Limb regeneration, prehensile tail, the ability to bury one’s self in order to get out of the elements. (Not unlike the roomy shell mentioned earlier.) Debuffs could include things like “Toothless,” where eating takes twice as long.
Finally, a post-threshold mutation for the Reptile that gives a naga-like snake lower half. Makes pants and footwear obsolete, but gives a bonus to swimming. (Probably prohibitively difficult, being that it changes legs. Unless you just lop one off or something.)