Here is an idea: use Mapsforge binary format for map data. Map sizes are quite small, especially useful for not to huge areas. Implementing HTTP Range requests might be an interesting exercise in reducing bandwidth usage.
Somewhat bizarrely I ended up doing exactly that as a uni project. Writing a parser for the Mapsforge format in TypeScript was a bit of a weird experience. The funkiest bit was trying to get the range request stuff to work properly offline via service workers.
My only regret is not putting more effort into getting WebGL working, I just used the plain canvas and it was a bit of a bottleneck.
