Let’s keep things simple two rules.
- No giving sentience, This is a no brainer issue.
- Let’s keep it to beings under the Animilia kingdom. “mutated virus/bacteria” is a common trope.
To start:
Let’s modify ants to have lungs.
Most insects are constrained by the amount of oxygen they can acquire through their exoskeleton.
Imagine how big they can get if they didn’t have that constraint?
Mhh, insects in general IIRC are also constrained by the fact they have exoskeletons: i.e. their internal organs are just in a bag. So they can’t grow too much or they just squish I think.
Imagine them being able to become vertebrates. You can have insects the size of cats I think before the osmosis breathing tubes they currently have are not a thing. Lungs would be the next thing I guess.
Disclaimer: I’m not a biologist and just make stuff up from what I remember from high school biology, and it was last millennium!