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?