I'm sure we all know from Subspace that Mr. Game and Watch is made up of Shadow Bugs.
What HCs do you have about how that affects him? Me personally, I always imagine him having less stamina in daytime, so I always have him spam the breathing taunt after each kill if it is daytime.
It does feed into a theory I've had that Mr. Game & Watch's universe is more similar to the Dark Realm rather than the Light Realm like most other fighters' universes. Specifically based on how well he meshes into pretty much any part of the Dark Realm vs how starkly he contrasts with the environment elsewhere.
Also the similarities between the Shadow Bugs forming primids and Dharkon's captured fighters strengthens this idea.
Much like how Dharkon being at more power than Galeem makes the screen visibly darken while Galeem only affects it marginally (if at all), I can imagine being in the Light Realm is harder on the eyes for him.
I can also imagine Mr. Game & Watch being affected differently from dark magic and light magic than most other fighters outside of battle (though for the sake of Smash this effect is reverted for consistency's sake).