It would be cool to import the JSON data exported to be able to add landmarks and change district names, rather than needing to regenerate a city each time.

I second this. I would love to make modifications to the city that I already exported. 

thirded

It will be implemented eventually.

Wuuh that would be amazing!

Thank you for your amazing work

Would love that. The reusability of adding landmarks as they are discovered or change. This generator is by far my favorite map tool aesthetically, and with that function I would cease to use anything else for cities.