I think you might misunderstand what I am trying to build. I am building a fantasy mash up of my favorite fantasy game elements and trying to give it the OSR treatment. The class building tools would be to build fantasy classes. Same with the race building tools. In Spycraft, campaign qualities are ways to modify a spy game; this would be ways to modify fantasy games.
That being said, approaching the design in two stages sounds extremely worthwhile. I could build the main part of the game, then go back later and show how people could build their own stuff, which would be much easier after doing it myself.