Damien Darhk is going to be the main antagonist in both the past and present since he's supposedly behind everything in both setting up to this.
Arrow did jump the shark with the whole Olicity shyt. But I am glad that may FINALLY be over, I felt that was the main reason for a subpar season. With that said, I am happy that the two characters in seasons 1 & 2 matured. And that is Laurel and Thea. As trying as it was, both of their character evolution paid off to me. So, I can expect them to be strong characters from here on out.
My prediction is that with Ray Palmer supposedly dead, Felicity is going to inherit the company since the transfer contract is in her name and since she was still TECHNICALLY VP. Through that is how Oliver is going to get his company back and the office that Ray been using and Felicity been borrowing to handle ops is going to be the new "Arrow Cave". But Palmer's death is going to be the reason why they'll return from their vacation.
Being that Damien Darhk is the main antagonist, and he particularly got beef with the LOA, I think Nyssa is going to conspire and betray Merlyn by making a deal with Damien Darhk which inadvertantly force Ollie to be involved.
In the flashback, since Ollie appears to be going to coast city, I predict it's where we going to get introduced to Hal Jordan and we get an origin story prior to getting the ring.