Crossovers happen in comics all the time, where characters stumble into worlds that look radically different and meet alternate versions of themselves. Such a thing probably won't happen with the kid-friendly Lego Batman 2 and the Dark Knight's ultra-violent exploits in Batman: Arkham City.
But, damn if Wonder Woman, Robin, Joker and Catwoman don't all look great when given the gritty grey-tone treatment from the Arkham City promo art. These would make great variant mini-figs. You listening out there, Lego?