app VS coffeosm

Woah, your vision sounds very similar to the goals of an open-source web app I started building a few years ago. The idea was inspired by Cities Skylines but I started with a 2D approach since everything is much simpler than with 3D. The idea was to help urban planners fill sustainability gaps in their cities by showing which areas needed access to amenities like bus stops, daycare, grocery stores, and schools. The approach, as mentioned is to overlay buffers surrounding existing amenities, dissolve those buffers, and find gaps. There can even be an interactive component to show the impact of placing an amenity in a neighborhood, like those emojis that radiate outward when placing a public service in Cities Skylines. The inverse problem can easily be accommodated with the app, helping people find desirable neighborhoods for their next home.

I'd be glad to jumpstart the open-source project if I can find any contributors to help build the project.

https://github.com/SustainableUrbanDesign/app

In addition to StreetComplete which others have mentioned, one of the lightning talks at FOSDEM this year was about a project called CoffeOSM, which allows you to scan your receipt from a coffee shop and add the shop to OSM.[0] It's still just a proof of concept, but hopefully it develops into something more as it's a very cool idea IMO.

[0] https://gitlab.com/netvandal/coffeosm