My thing?
I'd prevent cars from becoming so popular, which costed us actually-liveable cities and a massive amount of pollution and wasted space. I'd make public transport (buses, trains) and cycling, and even walking, more preferable to cars and motorbikes, like it is in Japan and the Netherlands.
Alternatively, I'd get rid of nicotine smoking, or change history to make nuclear power plants supersede coal and gas plants, due to being much less polluting and much more efficient. One of those three, I'd say, or a secret fourth option.
I'd prevent cars from becoming so popular, which costed us actually-liveable cities and a massive amount of pollution and wasted space. I'd make public transport (buses, trains) and cycling, and even walking, more preferable to cars and motorbikes, like it is in Japan and the Netherlands.