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?
Ok, take this with a grain of salt because I read about it ages ago in a dubious pop-sci book and my memory is shaky. One time, they tried to gene edit yeast to be able to survive much higher alcohol concentrations. There’s lots of good reasons to want to do this… Beer/wine is just about the strongest beverage you can make without distillation of some kind because the yeast dies. Making way higher ethanol yields just from fermentation makes biofuel way more viable. Stuff like that.
EXCEPT… It nearly escaped, and was able to survive on it’s own. Yeast is very ubiquitous in nature, so a wild yeast that can tolerate massive ethanol concentrations could conceivably have altered life on earth as we know it.
A cursory internet search isn’t turning up anything about this, but I’m pretty sure I read it in the book Everything Is Going to Kill Everybody, if anyone wants to look harder than I did.