Oh, sorry @outtaspace I missed your question. I hope it's not too late, I'll answer anyway, maybe you'll come back (I see you've disabled PM so I can't send you a message) or another person will find this thread from a search engine, on "how to create a fantasy map based on openstreetmap"!
You might have found some clues in my previous messages, but I'll detail here the complete process.
First, I've drawn my map by hand on a sheet of paper. It might not be necessary, one can create it from a vector based application such as inkscape (or illustrator), or even from a bitmap tool such as gimp (or photoshop). I've scanned and vectorised my drawings, and assembled them in one document. On my first attempts, I've exported my vectors to bitmap and used gimp to create the paintings on it. It looked rather ok, but was tedious to add new elements or more details. Also the picture was very big and slow to process.
Then I thought it could be great to use a vector only solution. Inkscape (or similar) was tedious too in the case I want to alter a generic element such as mountain, woods and such. And I wanted to design my world freely, without thinking to the final result at the moment.
Meanwhile, I was using OSM (and the josm editor) to work on the cartography of the area where I live. So it was natural for me to wonder if I could also use josm to design the area where I dream
https://josm.openstreetmap.de/
I don't remember how I converted from SVG to OSM. There are some online tools, or maybe I found a script to do that.
I've adapted the map to fit in the editor so the distances are coherent, it's useful if I want to design towns with houses and such. I've also used the Mercator projection. It probably means this world has to be similar to earth in size, but it's not a problem for me. It's maybe possible to design new projections but it's beyond my knowledge.
I've designed a CSS for my world so it won't look like a modern map (you can catch the different in a previous post). In josm you can use a custom css (called mapcss because it's a bit different from the css used in web design). With mapcss you can like in css import images and elements into the design, so if you change the source image (to improve it) it will reflect on the whole map. Exactly what I needed!
I'll try to post this mapcss somewhere soon. Btw if I post it somewhere, I could try to make its way into the official josm cssmap.
A picture of josm while editing my map:
josm_edit_ulbretelir.png
To get a better rendering, I'm using ceyx-mapcss see https://wiki.openstreetmap.org/wiki/Ceyx . First I've tried to use other tools but they were complicated to use (required to create whole map server). Ceyx is no longer developped it seems, but it's small and efficient. Maybe there are more now, I'll have a look. Ceyx can reuse the same mapcss for josm.
I convert my maps in png using the osm2png tool from ceyx:
And if you design for example some houses and such, you can see them in 3D using http://glosm.amdmi3.ru/ (see the video in the previous page, it was made from the same osm source)python2 ./osm2png.py -d world.osm -a -s 12500 -r style.mapcss -z 15
There are also other possibilities for building nice or complicated maps, such as qgis or http://kartograph.org/
I see also this tool can generate a procedural world based on OSM data: https://github.com/reinterpretcat/utymap