This idea probably sounds stupid, but I've just been thinking. The invincibility cheat is fine and all, but how about something like that, but you're still affected by the enemies? (as in recoiling and all that). I just think it would be kinda neat, though this may just be because I suck at certain games, especially without a lifebar (Mega Man and Castlevania come to mind.)
This sounds like a good idea actually. In the first Sonic game, if Debug mode was on and you got hit with no rings, Sonic doesn't lose a life and rather recoils like if he gets hit with any rings, and that continues for the rest of the game.