That is not a bad idea. I am not a mod of this game though. The only thing that could be a problem is the loadout and health that you enter the level with. The only way for this to be fair is to set the difficulty in the console (g_skill 0,1,2,3 - Recruit, Marine, Veteran, Nightmare respectively) and to run levels from the "DEV" menu because of the loadout. So if individual levels get introduced, they should have Any% and Nightmare Any%.