Before anything else I would want to say that this is my theory on how Arrow could've went and also it's not all about season 4, the theory also has to be taken back to season 3, when Oliver was about to become the next Ra's Al-Ghul. Despite what had happened by the end of season 3, I personally think that Arrow could've went a different direction which would lead to an awesome season 4.
Thea could've become Cheshire
Now as you all know that Ra's killed Thea and she died, but shortly after Malcolm suggested Oliver to take Thea's body to Nanda Parbat to be resurrected by the Lazarus Pit and then she comes back to life assaulting Oliver and then faints. What if it could've gone a different way. It could go this way, Thea faints and suffers memory lost after being resurrected, when she wakes up she doesn't know who she is or where she is, all she knows that she needs to get out of there (Nanda Parbat). After escaping Nanda Parbat, Malcolm goes into the room to check Thea and realizes she's gone. Weeks passed Thea keeps getting only small glimpses of her past and one of it is her killing Sara Lance and once she sees that, she'll think she's an assassin and proceeding her life killing to get what she wants and adopts the name Cheshire, since she doesn't know her real name. After that Malcolm becomes obsessed in finding Thea which leads me to my next point.
Malcolm becomes Sportsmaster
You know how in Young Justice, Sportsmaster is the father of Artemis and Cheshire and husband of Huntress? Well it doesn't have to be that way because since Thea is Malcolm's daughter and he loves her so much, losing Thea would drive him mad and would do anything to get his daughter back including killing. Since Malcolm Merlyn is trained by The League of Assassins, I think he would make a pretty solid Sportsmaster, considering he is actually an anti-hero rather than a villain. With that being said, Team Arrow returns to Starling City (of course my theory predicts that Ra's will double cross Oliver once Thea is resurrected, by knocking Oliver and Team Arrow out and sending them back to Starling City without them knowing while Oliver stays and gets brainwashed). Then it will go downhill from there leading to...
Roy Harper returns and becomes Red Arrow
Alright, pretty sure you've seen Young Justice so let me make it fast, in Young Justice, Roy Harper takes the name Speedy but after seeing how the Justice League doesn't trust him, he goes solo and then bumps into Sportsmaster and Cheshire where they replace him with a clone, and the cloned Roy Harper doesn't know he's a clone therefore taking the name Red Arrow, (but shortly after the real Roy Harper returns 8 years later missing an arm and takes the name Arsenal) so get this: What if Roy hears that Thea died but doesn't know that she's resurrected, who tells him that? Malcolm Merlyn. in order to find Thea, Malcolm would need Roy's help to make sure Oliver doesn't know that Malcolm is turning bad and Thea turns bad. Also Roy won't know that Oliver is actually brainwashed into joining the League of Assassins. After Roy taking the news of Thea's death he'll make a come back to Starling City, gets a new costume donned and takes the name Red Arrow, flies to Nanda Parbat to confront Oliver (Al- Sa Him). It would be awesome to see Roy going solo and returning to Arrow!
Diggle finally becomes Guardian
So this is another thing that could happen, after Team Arrow gets sent back to Starling City, all of them moved on with their lives (like I said before about Thea, Malcolm and Roy) Diggle will be continuing his work alongside his wife Layla and after months of being on missions, he hears that: Roy is back and wants to kill Oliver, Malcolm is turning evil and Thea becomes a killer. He dons his own suit and armor with the help of A.R.G.U.S. to serve justice and hunt down Malcolm, Thea and Roy.
Why would this work?
It would make an awesome plot twist and this could set up H.I.V.E. (potential big bad for season 4) very well in season 4 and of course once Damien Darhk finds out Oliver is the next Ra's Al-Ghul he becomes interested in meeting him and sets up some sort of rivalry between the two. DC rarely makes twists like this, compared to Marvel with Captain America The Winter Soldier revealing that S.H.I.E.L.D. is actually H.Y.D.R.A. This would make a very good move for DC and the Arrowverse.