You can now upload, change and download maps at Online Editor. It is still very far from complete, but enough to do some minor tweaks easily, like changing the world type from Greenland to Winter World. It also does some hidden magic behind the scenes, fixing a few things that can cause crashes or buggy behavior in The Settlers II.
Comments are welcome as are suggestions for highest priority features. Note that making an editor that shows the scenery like in game will take a lot of time to code so for the moment I’m pushing that idea into the distant future.