This would be really cool if it was properly worked into the game. It would be interesting to see the process through which an organism would gain this trait, since one of the most significant requirements of any trait in our world is that it must be useful through every evolutionary step, as shown by the evolution of the eye.
We can see that the journey from photo-receptive cells to the complex modern eye was filled with a series of iterative steps which built up on the previous evolutionary breakthrough; there was never half an eye, only specialised organs which led to the eye.
Using this principle, it would be really fascinating seeing how a player could develop “fire-breath” in a way that is useful through various generations. Perhaps it could arise from integrating bacteria which aides in extracting nutrients of the organism’s preferred food into a player’s digestive system, the initial expense being that the bacterium releases ethanol and sulfuric acid, which can have some minor detrimental effects on an organism; the player could then create a new storage organ, which could initially help with the safe storage of the released gases and later transition into an organ system which helps weaponise the gases into a fire-breath with some minor innovation with the player.
The possibilities could be endless with a system that intricate, which allows for outside the box thinking to be rewarded with some cool evolutionary traits. It would be a huge hassle to implement though, but if achieved, it could truly make Thrive revolutionary.
P.S - I think there should be a fun thread in this forum in which members could brainstorm ways of creating untraditional traits through an iterative process in the game. It could really help define certain game concepts further.