At the moment there are a few people working on map editors that allow you to add maps and custom tilesets(new dropships/UFOs/mapblocks for the RNG to pick from), I think getting this to a point where it's fairly easy to work with and bug free would be the first step before putting something together which you could setup of custom battlescapes that you can drop items and units into.
If you are so inclined as to make one, my thought would be to figure out how the "battleGame:" section of the savegames work and work from there creating custome savefiles that open up to the battlescape you want.