What is something small that could be changed that would drastically improve medicine and healthcare?

Make health tests mandatory every 6 months or 1 year in every country.

Here is a complete list of health test I would suggest for someone annually.

Healthy citizens actually save a lot of money and are more productive.

Health score card should be given to every citizen which gives him a preference over other people who are careless about being healthy. Something Like a HEALTH CREDIT SCORE. Yes, this should be a criteria.

Once preventive health becomes priority cost of healthcare will come down automatically as people get less sick, less sick means less demand for hospitals, less doctors, less medcines.

Like all systems, it has loopholes like what about poor ignorant people in polluted environment, war zone, but yes as i said this should be like a CREDIT SCORE which is scored on the person's environment.

Genetics - Yes should be taken into consideration while making a HEALTH CREDIT SCORE. To prevent low scores for people who are congenitally diseased, handicapped.

It is susceptible to loopholes, so it has to be strengthened over a period of time but sooner or later, imagine we a have a Universal HEALTH CREDIT SCORE.

People who smoke, drink, abuse their health will be in trouble.

This can also lead to people being jealous and poison or affect other people's health. So strict punishment should be levied like how sports players are punished for substance abuse.

But overall imagine if 90 percent of people in world are healthy. As i said system has to be reviewed, be incorrigible like Credit Score so that there is no scope for arm twisting from powerful people.

So develop a Health credit score, keep reviewing, in a few years, we should have a system which is almost perfect, but always improving.

Finally, we have a healthy world where cost of healthcare becomes cheap.