I love the set piece game idea, but an added level of flexibility would be to make an otherwise "illegal" move that would only take effect if the opponent approved it.

As it is, we can do this by going into Game History and choosing Start New Game from this Position, then alter it with Set Piece Game and starting from there... but its fairly disruptive to start a new game every time.

I'll confess that my ulterior motive for suggesting this... is to let me and my friends play Knightmare Chess online. We love playing chess, but now and then love a little zany chaos, but there's no good forum for it anywhere.

agreed

Even Fisher Random - with proper FR castling - will be possible with your proposal.