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?
The reason you don’t really see animals that can photosynthesize (other than microbes) is because you don’t actually get that much energy per unit of area. Think about how much area a cow has to graze vs the surface area of the cow itself. And much of the cow’s surface isn’t even facing the sun.