We had a variant we played in the coffee house some years ago.
Only once, every game, each player has the right to insist that his opponent tales back his move, and make another legal move.
Some amazing tactics came out of this, you had to make sure you have at least 2 ways to escape a check, otherwise it might turn into mate :)
As I mentioned in my first post this thread was set up tongue-in-cheek and we certainly are getting some fun ideas for new rules lol.