A Batman villain, Roman Sionis, better known as the Black Mask, has already made an appearance of sorts in Gotham: a character named Richard Sionis, dubbed The Mask, appeared in one episode of the first season. They didnt use his full name or proper alias though, nor did they really showcase all of his abilities, so there could still be a chance for the Black Mask to appear elsewhere, if the powers-that-be at DC allow it. Hes generally involved in the criminal underworld, so he could feature in Arrow as the head of a crime-organisation, spreading fear throughout the city, while his skills as a marksman as well as a fighter would surely test Oliver.