For instance, retooling a supers game like Heroes Unlimited to the world of Strontium Dog from the 2000AD comic? (i.e. it's the far future; all characters are mutants, whole generations warped by strontium fallout from a nuclear war in the distant past; the characters are backed with nifty tech; all characters have one power; the power range is limited; there's no Invulnerability power; the characters all have a job working for the law as freelance bounty hunters, and norms hate them).
Conversely, have you kept the setting and imported your own game mechanic instead because it works better than the rubbish game engine they have?
IOW, how much do you like to mix things up in your own game?