Wow this is something I never realized I wanted until now. Seems like it could be a cool hackathon project, plus getting to learn more details about how google maps / bing maps / openstreetmap do their zooming.
I’ve played with a bare OSM server before to try and get an osm based map working. Ends up you just have to scrub all the existing geometry and draw it from scratch, but it’s so-able. Worst problem I had was trying to generate realistic height map bit maps.