Although there are 3 tools with overlapping features, each of them has different benefits for different purposes, like Homer's 3D approach for example is great for finetuning and giving LIFE to the map (just look at the stunning beauty and detail of the 3DE island)...
Using this one generator I'm dreaming of won't be enough. It might be a really great help for large areas of woods, basic road networks, basic city shapes, BUT for nothing more, as not any GIS-data is either way freely available on the net OR has been archived (like walls, fences and such)
Therefore, after the use of the generator, one would have to really look on the map and give his personal touch to it...rotate objects, add stuff like bus-stops and such...i.e. making it look real and thus creating an awesome immersion...and for THIS purpose your's and Homer's tools would have their finest hour then!
Furthermore, for a complete fantasy island one wouldn't use the generator, I guess...
So basically, what we are talkin about are at least 2 tools (for realistic maps):
1. A generator using large scale GIS information, working on a routine like the "import terrain from xyz"-option in Visitor for creating the "big picture"..
2. A detail-in-game 3D object editor for adding "life, immersion and the editor's personal touch"