To make it better, what would it be?
This can be a bit like the Twilight Zone world. You might not realize the consequences of your choice until it’s too late. So feel free to play devils advocate.
So what would you magically eliminate that would make the world a better place.
Mosquitoes? COVID?
For lack of a good term to express it, self-bias or lack of self-objectivity. I’m convinced nearly every single problem on earth would be fixed if we could think with almost no bias filter and see things from someone else’s viewpoint almost as easily as we see it from ours. Think of climate change, war, politics, the economy, relationships, pollution, almost everything that could be fixed if people could tackle a problem as an objective issue instead of me-vs-you.
I’ve come to see that almost every problem in my own life, for instance, stemmed from my skewed and subjective thinking.
Nice one. I find as I get older, I get much better at seeing problems from both sides. I sure as hell wish I would have had that ability in my 20s.
Hate.
Faith. It’s the opposite of reason.
Craisins.
I am a custodian at a school where they had them out to students by the bagfull but do nothing to clean them up when said students pour them onto the carpet, then step on them, turning the carpet into a giant Fruit Roll-Up for ants. They don’t vacuum out worth shit, and carpet cleaning is a major bitch.
Cancer.
Mosquitos.
ETA: Bed bugs…fuck them. Awful creatures.
I’d say tangential. We take various things on faith ALL THE TIME, be it in politics, personal relationships, science, or just the way we live our day to day lives (such as trusting that the salad we are eating for lunch doesn’t have nasty things in it). Yeah, sometimes that trust is misplaced, but if we were determined to only embark on those activities (incl. mental ones) where we knew ALL the relevant facts for certain, we’d spend all of our time rocking back and forth in the corner of our bedroom, too terrified to even move, until we starved to death.
Of course you likely meant it in a religious sense, but even there that would be a simplistic way of looking at those two concepts.
Politics. Or ticks.
I was going to say chewing gum. I hate finding it on the bottom of my shoes.
Dental cavities!
Humans. But gradually! We’ll need enough time to set the pets and other captive animals loose.
Deer flies. I hate those damn things.
There goes reproduction. 
We’d eventually evolve again.
I will be specific, hoping to limit collateral eco damage- Aedes mosquitos in the Americas- where they are invasive. That should be fine- invasive, and there are still plenty of other mosquitoes for birds and such.
I’d say all invasive species- but that would eliminate most humans. 
Say- every invasive species in the areas where they are invasive- and that came in in this century.
I cannot find the cite now (sorry) but I think I read that mosquitoes could be eliminated from the planet without any ecological repercussions. Yes, some things eat them but not enough to impact ecosystems if the little bastards disappeared.
From a pest control company: “Mosquitoes are considered biological control agents, meaning they help keep insect populations under control. Mosquitoes also pollinate flowers and serve as a food source for larger animals such as birds. Mosquito larvae are food for fish, frogs, dragonfly nymphs, and birds that feed on water insects. Adult mosquitoes are eaten by bats, birds, spiders, dragonflies, and other insect-eating organisms. They also eat the nectar of flowers and fruits such as mangoes. Mosquito larvae have a symbiotic relationship with some small crustaceans, such as copepods. During the larval stage of the mosquito, the copepods live within the larva until it emerges from the pupal stage into adulthood when the copepods leave to find another host.”
Do Mosquitos Have A Purpose To The Ecosystem? | Lookout Pest Control.
Dementia.
I can see problems if we magically eliminated aging. That would lead to increased population pressure on our planet’s already strained resources.
But I feel the mental decline of aging is far crueler than the physical decline. It’s terrible when a person is still physically there but their memories and personality and skills have all disappeared. It would be better if people could remain mentally sharp right up to the end of their lives.
The letter “C”. We have “S” and “K” already.