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

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

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.

Low back pain eliminated in all humans.

Eradicate all mosquitoes

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.

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

No more cavities!

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

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.

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

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.

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

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.

Advertising is illegal

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

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

Make everyone understand basic propositional and predicate logic.

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.

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.

Every child born has a flat increase of 0.0005% higher IQ, and is also 0.0003% more altruistic.

I really should have done the math ahead of time. That'd mean an increase of like 70k IQ per year.

Bring back the concept of shame.

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.

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

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

"You are a human too" for all people.