If you were to replace all the art in the default FS2 VP files with a completely unique and original universe, you could stuff all that on a CD and sell it, completely legally, with a few restrictions:
- No use of volition files, ie POFs or VPs.
- You could include the Launcher, but not FS2, FRED2, or POFCS. (The latter depends on the legality of reverse-engineering file formats)
- Pieces of art specific to the FS universe in the mediaVPs (ship skins) could not be included but, AFAIK, you could include missile trails and such as their likeness would not be owned in any way by Interplay or Volition and wouldn't have anything to do with the FS universe.
That of course is based on my knowledge of US copyright laws.