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?
According to my sister-in-law’s then boyfriend (who was a biologist or an insect guy or something), spiders and insects can have lung-line organs called book lungs. So they have options. Creepy buggy insecty options.
Oh right, I’ve read about that on coconut crabs I think. There you go, they are already half there.