For fictional terrain, the only "obstacle" is that you need to generate the data to use yourself. You will need the following:
* Airports: These are normally the easiest to make, using X-Plane's WorldEditor
* Shapefiles (aka Landcover): You can also draw these rather easily, e.g. using QGIS
* Elevation data: This is a bit more complicated, but thanks to SurferTim's HGT editor (
viewtopic.php?f=5&t=36143&), this should also be not too difficult, best would probably be to download an existing tile of SRTM data and then modifying it to your needs.
See also
https://wiki.flightgear.org/Using_TerraGear for more information on how to use this data to build scenery.