I wish for one electron to disappear from every atom. The net result would be that all atoms would now have a positive charge.

True, it would not only end all life on earth, but also destroy the entire earth. But everything would be positive.

Simple. True empathy for everyone. Literally feeling what others feel

I’m trying to focus this answer on something that seems like a really small change:

I wish everyone is slightly more empathetic.

I feel like this could give us a lot of small nudges toward being better people and a better society. I wonder if a small nudge could end up having a profound effect.

Everyone religious wakes up tomorrow realizing that it's just a social club and none of the god stuff is real.

Low back pain eliminated in all humans.

Eradicate all mosquitoes

People can no longer share or post something on social media unless it is objectively true.

Everyone is gifted with the ability to control their own fertility. You're only fertile if you want to be. The only chance for pregnancy to occur is if both partners want it to.

I imagine that would cause a severe population decline, and I'm fine with that. There's too many humans on this planet already.

No more cavities!

Make everyone's interpretation of "small thing" slightly more similar.

A ubi funded by a hefty carbon tax on big polluters.

Every one gets a strong moral compass that they can't ignore.

Sure we won't all have the same morals but I believe that most bad things in the world happen because people ignore morals and act selfish and only a small part of our issues stem from actual moral differences.

Edit: Seems I am much more optimistic than I thought.

Pull a gazilion tons of carbon out of the atmosphere, crystallise it into gigantic diamond shards, and drop them from the stratosphere onto the 95th percentile by wealth in each country.

Metacognition becomes routine for humans. We are able to better de-fuse from our thoughts, and recognize them not as reality but as thoughts about reality.

Normalise the phrase: "I don't agree with your opinion and that's fine"

The invention of a small easily producible power source that never runs out and has enough power to power vehicles/planes/vessels of all kinds.

Make everyone understand basic propositional and predicate logic.

Not a very small thing maybe, but: All people gain ability to instantly recognise bullshit (at work, in media, interpersonal relationships, etc)

Every figure leading every cult of personality is now seen as an imposter pretending to be themselves. For example, people think some orange-faced lunatic keeps trying to get on stage at a Trump rally, but Trump himself just disappeared.

OP asks for small thing, most replies ask for huge things.

I guess the opposites of small thing and biggest positive impact on the world makes it hard to answer.

To remove the desire for power over others.

Advertising is illegal

Not sure if it's a small thing but I think I would change it so good deeds make a bigger impression on people than the bad.

It's a bit of a rainbow sunshine kind of wish but it's human nature to remember bad things easier than the good, it's a survival instinct to protect ourselves. But with all these conflicts and discrimination, big or small, if people don't let fear dominate, everyone would cut each other more slack.

Of course this requires everyone to have the same mindset to work, that's why I think making that small change in human nature at its core is the only way, and only a genie could pull that off.

A black hole about 2cm across filled entirely with protons.

A deadly virus that kills assholes

Elimination of the tribal parts of the brain that want to go to war.

After decades of sci-fi/fantasy entertainment to prime us, the primal part of the human brain that reacts to in-group and out-group members suddenly changes in every human and we start reflexively and unintentionally classifying all earth life as friends and space/environmental threats as enemies.

Humanity immediately gets serious about climate change, CO2 reduction, and the like, but we also get way too zealous about deploying space lasers.

People regain the ability to agree to disagree, and stop hating each other over political issues.

Bring back the concept of shame.

Initial thought:

Everyone is nudged slightly closer to what they need to live comfortable lives

For people with very low quality of life, their life gets slightly better For people with very high quality of life, their life gets imperceptibly worse (but then also slightly better as the world around them has improved, since they make up such a tiny fraction of people)

"more realistic" thought: The human brain is slightly less prone to addiction - however slight this adjustment needs to be to be considered a small thing.

Less social media addiction, less harmful substance addiction, less sugar addiction, less gambling addiction.

"You are a human too" for all people.