Sara Lance (Caity Lotz) and Laurel Lance (Katie Cassidy) as Black and White Canary respectively. Because I don't want the Arrow writers to kill either and I'd love for them to co-exist in Ollie/Arrow's future.
UPDATE: Well my thought process is that Nyssa relinquishing Sara isn't good enough for her father. So if Nyssa refuses to bring Sara back, then he forces her to get a replacement - Laurel. So brainwashing happens... Laurel is evil for awhile.. blahblahblah.. and at some point they're just ass-kicking sisters fighting side-by-side. Taking on the likes of -- (pretty please Arrow Producers) Lady Shiva ! Yeah - one can only dream.