Thanks for the quick thoughts/feedback.
We'd have to work out some issues for the blank map. It could be that we start off with an empty map that can be loaded, or there is a quick wizard process that gets a map maker to that point. TBD exactly, main idea though is that the tools would be moved to in-game rather than be completely external.
Glad the idea of map configuration hosting sounds good. Implicit to that idea, is that the entire map would be stored on the server, in DB!! No more github manipulation or uploads. Hence the asset split is particularly useful. If the UI tools are good, then map makers will have a UI to select their assets by image and it will be written into the configuration properly without anyone dealing with IDs. Part of the goal would be to decrease the manual file editing. I suspect a fallback to file system will almost always be there to support offline map making.
I was thinking that the potential new map database could write out to a github repository and check in the contents, largely to serve as a backup. That's a detail for now that is less important compared to the other main ideas.