I'm not going to sit here and say it's our biggest problem as a society but it's worth looking into: we should be building fairly dense cities, with no gated communities and no car-dependency.
The city block grid systems most cities and many downtowns use work exceedingly well on relatively flat terrain. If you're on mountainous terrain, you can do what Chongqing is doing, with the multi-level shenanigans, or build along terrain, of course shaping it as you go. I would say there's room for exploration here but people's lives and livelihoods depend on this stuff so, do so with care.
There probably shouldn't be any building in a city less than two stories tall. It's wasted space. If you have some large warehouse or depot, you can build on top of it -- run a street on top of it and build more buildings there!
I'm a big fan of mixed-use buildings with public destinations (libraries, cafes, stores, etc) on the first floor and residential or offices on the floors above them. I think we should do more of those. It's a really clever and wise use of land. Your business probably doesn't need to have an empty roof. Build apartments on top of your Walmart and give people a place to live.
Private cars should be completely banned from cities. Make sure there's no need for them, set a time about a year out before it takes effect, so people can go sell their cars.
We should be able to reduce "car" lanes to one-lane one-way (or one lane each way), and that would be for buses* and delivery vehicles. Add loading zones on each block, possibly shared between buses and delivery vehicles -- I'm not entirely sure how these would be best implemented yet, especially to not interfere with bike lanes. Basically 15 minute parking for non-personal vehicles, but it probably needs to be next to the sidewalk especially so that people waiting on the sidewalk can easily get to the bus.
Buses are best suited as a gap-filler between other methods, especially during the development or repair of a line of fixed service, or as an overflow service, particularly during events and emergencies.
I should look into the differences in use a bit more, but:
Streetcar, tram, trolley, and bus service seems best suited for travel between nearby neighborhoods, or to shorten longer journeys in the same neighborhood (going from one side to another)
Subway service seems best suited for going from one side of the city to another
Biking and walking seem best suited for travel within the same neighborhood
and an e-cargo bike seems well suited to situations where you want to carry a lot of stuff
Cities should find some way to both give local shops a bump and disincentivize chains and franchises. Maybe a literal tax on chain businesses (restaurants, fast food, grocery, etc), defined well, that directly funds local shops, and/or is otherwise used to help businesses based in that city develop, open their doors, and stay alive. Maybe chains should, on top of that, be subjected to a neighborhood vote on whether or not that chain should be allowed in, before they can be brought in. It will take some effort to define those "chain" stores to include stuff like Subway, Dominos, Starbucks, Papa Johns, Walmart, etc etc precisely enough, because depending on how you define it there are plenty of loopholes. Basically no external business should be enforcing their rules on those businesses. No requirements of certain branding/advertising to supply, no defining retail prices, no defining what can or can't be on the menu, or deciding who is allowed to be employed. The business itself would have to be registered as being based within the city, so the ol' Delaware tax trick would be void.
We should also be getting rid of parking minimums. We need parking maximums instead, honestly.
Suburban developments, plagued by and developed to cater to the automobile, should be completely outlawed and we should begin destroying them posthaste.
The really tricky thing though is making sure the people who live there don't end up becoming homeless. As far as I know, when a given building gets redeveloped by a landlord, the people who live there just get evicted, and usually (but not always!) given some time to find new housing. And in this economy? Good luck. They should be guaranteed new housing that matches, at base, what the house they were already living in was like -- particularly number of bedrooms and space, but also just general quality of life stuff, like having the same built-in amenities, like you shouldn't be forced to move from a place with AC or a shower to a place without those things. It needs to be no more expensive, too, and there need to be options within that guarantee of different places nearby that the offer applies for. You should be able to find somewhere else to live, at least temporarily, and given a good offer to move back to where you were, no consequences of accepting or denying. This is an area I'm less familiar with but definitely concerned about.
My ideal map has a sharp cutoff at the edge of cities, where it immediately turns into farmland, greenspace, etc. This is probably more of an aesthetic thing than anything else, just for my brain to comprehend where the edge is, but it also makes clear the total elimination of suburbia.
All of this is inspired by my visits to my grandma's mountain town, and more recently to Raleigh, as well as my frustration living without a car in a suburban subdivision in the Raleigh area. Basically I want to move to some kind of city because living out here is incredibly isolating and boring.