I suggest keeping the features minimal and avoiding a “too gamey” look. Stick with a simple, non-RGB UI, those flashy designs are overused. Try using one or two unique accent colors while retaining the clean, black-and-white “nothing” aesthetic.
I expect in the gaming mode of a nothing device is:
Notification Blocker
Do Not Disturb Mode
Accidental Touch Protection
Dynamic Refresh Rate Control
In-Game App Pinning - Prevents accidental app-switching or home button presses during intense moments.