Is Ashley Scott's Huntress from Birds of Prey.
What may be the most important bit of casting news came when it was announced that Tom Welling, who portrayed Clark Kent for ten seasons on Smallville, would be back to reprise the role in "Crisis on Infinite Earths". Welling will Brandon Routh and Tyler Hoechlin as the third actor to play Superman in the five-part event. Just a day later, it was confirmed that Michael Rosenbaum's Lex Luthor, unfortunately won't be making an appearance.
With Welling and Durance in and Rosenbaum out, the next most important Smallville character with a reasonable chance of being in "Crisis on Infinite Earths" is Oliver Queen, played by Justin Hartley. Hartley's Green Arrow was one of the only superheroes other than Clark to be a main character on the show. Hartley recurred through season 6 and was a series regular from season 8 up until the end of the series. Oliver was a key ally to Clark who played a significant role in his journey to becoming Superman. Oliver was also the founder and original leader of Smallville's Justice League.
Oliver's importance to Smallville is a good enough reason for him to be in the event, but there is at least one obstacle standing in the way: last year's crossover. Smallville's Green Arrow appeared to be among the casualties of "Elseworlds" in one brief shot. However, though this was a clear Smallville Easter egg, this isn't something that can't be walked back, especially if "Crisis" has a good story to tell with Hartley's character.
One positive sign is that Hartley has said that he's open to appearing on Arrow. There's also the fact that his character would add something else to the show that has nothing to do with Smallville: a second Oliver Queen, played by an actor that's not Stephen Amell. Seeing the two Green Arrows meet and how they interact is a Smallville-Arrow crossover have fans have wanted for years, and now it's finally possible.