As something of a misanthrope, especially today, the only "ideal city" I can visualize would be devoid of humans.

Canada.

Like Austin TX in the 70's and 80's. It was great while it lasted.

Green energy, zero guns (only select police officers would have access to guns, like they have in England), and proper singlepayer healthcare. The city would have residential areas on the outskirts that feed into the commercial, retail, and industrial districts by mass transit, with no cars allowed within those districts to reduce congestion and pollution (people can use their cars to travel to mass transit stations if necessary). And the homeless would be taken care of fairly and humanely with three square meals a day, medical care, education, and job placement programs to help them. I guess I'm a progressive leaning toward socialism, sue me

Sans hoooomans

To me, this question is a no-brainer. I have been a proponent of the Venus Project for years.

Singapore.

I don't poll by strangers. Specially about ideas that are 3000 years old.

It's been way, way too long since I last read The Republic to answer confidently, but it would definitely be a complete restructuring of society and government. I don't think it would be feasible today, though, if it was ever possible at all.