3) Talia Al Ghul
Talia al Ghul has been a very interesting part of Arrow, not least because she’s turned on her former pupil to align herself with Prometheus. The last time we saw her, she was willingly handing Oliver over to the villain to be tortured at his hands in a bid to get revenge on him for killing her father (back in season 3), but what else has she been up to? We know a confrontation between her and her sister Nyssa is on the way, but there’s no doubt a chance that Talia could be revealed as Vigilante.
This is another one potentially hard to explain, but Vigilante has a lot of skills and it’s possible Talia has decided to show Oliver how a vigilante should set out to protect their city due to her believing he has failed as both her student and in his mission. What her endgame might be is ultimately hard to say, but her father’s demise would probably come into play in some way.
Then again, should Nyssa end up taking her down in battle, then we’ll know that we can take Talia off the list of suspects.