How about randomly placed "wild" pieces? Those grey pieces could be randomly placed at the beginning of the game by the software (just a couple of them). Then, for the rest of the game, either player could use those pieces as their own. They'd never change status, always usable by either player. A grey located diagonal from the corner would definately add a significant aspect to the game. :-)
(hide) If you don't want other users to know what you are doing you can change into cloak mode in Settings (paying members only). (pauloaguia) (show all tips)