I like the idea of undesirable in category mutations being linked to desirable post-threshold mutations. As per ectothermic.
How about this one for Chimera?
Chimera : Fight or Flight.
Pre-req : Extreme Metabolism, High-Adrenaline
Description : You’ve become hardwired for self-preservation by a simple and insatiable need to subdue your constant hunger. Bestial tenacity slumbers at the edge of your awareness, poised to flood your body in times of need.
Effect : When starving to death you gain a strength boost. When heavily wounded you gain a brief but sizable movement speed boost. In essence you can kill better when hungry and flee better when close to death.
Also, Alpha.
Alpha : Übermensch
Pre reqs : I’m not entirely sure here, maybe a collect em’ all of the prime stat mutations?
Description : You are the Ideal. The Goal. The Over-human. Where lessers would see a cataclysm you have found the crucible with which to temper your superiority and shed the failings of your ‘peers’ like so much slag. You will tolerate nothing less of yourself.
Effect : When a skill goes up in level your morale sky-rockets. When a skill drops a level it plummets ((Though to a lesser extent.)) In essence it’s a self-improvement addiction that becomes harder to sustain the more you perfect yourself.
EDIT*
Yet another suggested post-threshold
Arachnid : Urticating hair
Pre-reqs : Chitinous Plate
Description : A fine phalanx of barbed setae rise from fragile bases along your stiff, arachnidian, exterior. Even the slightest provoking touch is sure to slough a choking cloud of lancet-like hair into your vicinity.
Effect : You release a sizable cloud of urticating hair when taking damage from any source. ((Some monster-type spiders should get this too.)) It severely hampers vision and breathing of anything that need breath or see, and is generally painful for creatures without this particular adaptation.
******ANOTHER EDIT
Lizard : Vomeronasal Organ
Pre-req : Forked Tongue
Description : Your forked tongue has evolved into an extremely sensile olfactory probe, its ever flicker a brush-stroke upon a mental canvas of the world around you.
Effect : Simply put the player can ‘see’ the various odors floating around cata. I’m somewhat on the fence as to if this should be A) access to the scent-map B) Detection of invisible or otherwise hidden life-forms or C) All of the above.