like his original costume but sewn together by a crazy person from leftover material
Other than using his mouses what would be his hobbies?
I like the “disease being the great equalizer” take someone posted and I’m coming around to home using rats and mice as surveillance and spying equipment.
The former could make for an interesting team up with Doctor Poison and the latter as an agent in Doctor Cyber’s cabal.
Rodents are a hardy, intelligent survivors as a species, so I might make his origin thematically fit that.
As for hobbies, I’d make him sodoku and super villain cataloguing aficionado.
How would he be a major threat to Wonder Woman, without WW jobbing to him? Squirrel Girl is a comedic character, really there is no way for her to beat Dr. Doom or Thanos, but the writers do it for a gag. That doesn't work for a villain the way it does a hero, you couldn't have Mouse Man defeat the clearly superior Wonder Woman, without fans thinking it demeans WW.
Squirrel Girl works because she is a silly hero, the audience roots for her despite her goofiness, Mouse Man is a silly villain, the audience just wants him to be humiliated as a joke.
One of the ways to make him a capable villain is he knows he isn't a strong villain or a very intelligent one at that. However, he is good at blackmail. He made intelligent mouse and many other rodents. He is able to use that to scam people out of money and somewhat give some more power villains information. That he is more of a coward villian working under others or playing both sides. He runs away when he dones it's time to give up.
I think the goal is just to be a threat to the people that Diana cares about. he doesn't have to be a direct threat. he didn't shrink himself to punch it out with people. that's what the mice control is for.
I see him more as a minor nuisance. Not a major villain.
I'd keep Mouse Man exactly as he is. Just use him as a joke character.
You can still make him a joke villain but it's the way you write him. For example, it should never want to be in fights only thing he does is use his rats to give information to other villains. He never tries to get into fights.