That is called "handicapping" and no plans, or desire, to do that. The only approved way to do it is to make your own scenarios, as you suggested.
The simple reason is that I don't want to join a server, on good faith, and realize I am Juggernaut or Tiny Tim. A custom scenario (hopefully named, Handicap DM) would clue me off to this effect, whereas a simple Convar would not.