What’s your specific use case? I would base it off OpenStreetMap as much as possible - building a whole map and keeping it up to date is a lot of work

If you enjoy building it, 10/10 great idea

using GIS?

I looked at getting gis data from my local county. Sent an email and all I got was crickets.

It all depends on where you are at and what's available to tap into, as far as gis is concerned.

I tried to make a tree location map using phone gps meta data a couple of months ago. What a mess, the precision is too low. You can buy precision gps receivers, but they are expensive. It was a great exercise though. Now I know why trail navigation apps fail around closely spaced switchbacks in mountain areas. The gps data is too imprecise.

But maybe you know more than I

There are already dozens. https://gisgeography.com/google-maps-alternatives/ How many millions of dollars do you want to spend on your project before you run out of money?

It’s closer to 1 than to 10 (it took Apple with all their billions something like five years to get their maps usable outside of silly valley).

But if you can use OpenStreetMap as a baseline you may have a chance.

There is nothing to it. I once made web.py-server that combines maps from two countries. While kayaking. It was too annoying when you had to switch programs to see what is on other side of river. Unfortunately they do follow Google tiles standard anymore.

Originally this program was running on Python For Symbian, which is historic and funny. https://github.com/timonoko/webpy_map_server

Tiles.py seems to be how you decode other systems to the Google-tile system.