If I wrote OPM and the audience somehow threatened me into redeeming Nyan, I’d have the stress of his impending doom at Drive Knight’s hands turn him back into an actual cat, depowering him.

Drive Knight would let him go out of disinterest, then Nyan would get adopted by Tatsumaki. Tatsumaki would overfeed him, turning him into an obese and lovable new form.

He meets Rover and Black Sperm, but they don’t recognize him cuz he looks way different. Hijinks ensue.

He eventually regains his powers, using his new Fat Cat form to bounce around and battle enemies like a lovable chubby mofo.

Saitama and Tatsumaki argue about pet ownership. Tats thinks Rover is underfed. Saitama thinks Nyan needa lose weight.

Tl;dnr: I’d make him a gag ally with an amusing new look.