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?
A malicious gene drive. Basically, use molecular tools to ensure that a gene is always passed down from a parent to its progeny, regardless of the other parent’s genetic makeup.
Many choices available: propagate resistance to a pesticide for mosquitos, guarantee Huntington’s disease in a family, or crash a population of beneficial species by reducing fertility, to name a few.
I loved the movie children of Men. It was very interesting. You might like it too. https://m.imdb.com/title/tt0206634/
All life with gills learn to breathe air without water, and suddenly all marine life is competing with us for land space
In a push to deal with micro plastics, bacteria are developed to be able to break down plastic. Eventually, it gets into the plastic installed for a purpose and starts breaking that down.
Amaranth that can grow on sand and be watered with seawater
Edit: ops, only animals? Make bees able to break down any kind of sugar into honey
