Under the synopsis for Arrow Season 2 Episode 21, IMDB had this written:
"Oliver gets a business opportunity from a famous billionaire in the neighboring Gotham City."
I first heard the news while reading a review for last nights episode, and geeked the bleep out. However, Oliver Queen himself says something else. Taking to a post on Facebook, he simply said "No."
Another episode had a reference to Scarecrow making an appearence. Clearly, someone is trying to get us fired up. I'd love to see Batman on Arrow, but I don't think the WB Movie Execs would!
I'd say we have a while before a big-league character such as Batman shows up. However, I considered Flash a big-league character, so its possible for a Batman appearence. Perhaps, if the powers-that-be decide the TV DCU and film DCU should not crossover, we'll get a version of Batman for the Arrowverse.