First and foremost, we'd need to convert everything to clean energy sources, like renewable and nuclear energy. We would need to establish a strong system of public transportation and also adopt the use of electric cars all over. Along with that, we'd need to build car charging stations (like gas stations) for said electric cars. We'd need to reduce the amount of concrete and cement used in our streets, sidewalks, buildings, etc. A way to keep cities cooler and to keep the air cleaner is to have green roofs everywhere, it would also help minimize the carbon dioxide. A sustainable city should also be designed in such a way that navigating it with a car isn't an absolute must, it should be walkable and bikeable. And these are only a few things, there is so, so much more we could do to make a city more sustainable.