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?
99% sterility. Virus transmitted. Long incubation. People would freak out.
viruses are not animals
I loved the movie children of Men. It was very interesting. You might like it too. https://m.imdb.com/title/tt0206634/
Depending on what causes the sterility it could be managed through diffecountries. It’s like huge spark to class warfare instantly. especially for less well off counrries.