Arrow is quite a controversial series and that is about as nice of a way to describe it. Now I'm not going to go into a full-length description of how I'll fix Arrow because that would take forever. So I'm just going to do an abridged version of the topic.
To start things off I'm going to kill Malcolm off in Season 1. If he died then he would have made a better impact in death than in life. Also his poison wouldn't have seeped into future seasons.
After that I would have developed Oliver into a hero who'd kill but only as a last resort. And the death of Tommy would have triggered that. Everything in Season 2 would still be in full effect. Although I would change it that, while Laurel would still be shocked in Sara being alive, she'd gradually try and make some attempt in rebuilding their relationship. Laurel will also have a side arc of her rising in political power. Because not every character needs to dress as a vigilante to take a level in badass. Also, there were hints of this but became dropped because...reasons. Because in my version the female characters aren't just token damsels in distress. They do shit.
Outside of the Miuraku Arc there won't be super powered individuals in Arrow unless I'd want to use that as an excuse to do a crossover involving Green Arrow and Flash. Arrow will be used to showcase street level heroes and villains. Much like how Marvel's Daredevil do their characters.
Thea would have her own side arc. As she's working for Laurel she develops her own detective skills. She'd also be getting her own share of training from Sara. Because, despite her investigation, she knows that she'd get her ass handed to her in a fight. In fact the first scene of the second season would be Oliver revealing his secret to Thea. It would be awkward but it would inspire her to want to do some good also.
I'd also scrap the whole Thea being Merlyn's daughter subplot. That was shit.
I'd keep Felicity but she'd be used in small doses. While she's a good hacker it wont be to the extreme levels that the show did. And, personality wise, she's close to her seasons 1 & 2 character. Also, she and Oliver won't be a couple. That was how this whole downward spiral got started. Oliver and Felicity work better as a sibling like duo. And that's how they shall be.
Oliver's primary arc would be maturing his character, he makes mistakes but he'd learn from them. It's his way to atone for how he failed with Tommy that resulted in his death. And his past history with Waller and Slade made him savvy enough to never to an enemy mine situation with a villain.
By season 3 it would start with Sara fending off an assassination attempt. Orchestrated by Ras Al Ghul. Who holds a rather zero tolerance towards running away from his guild. Despite his daughter's attraction towards Sara an example needs to be made and Sara needed to be punished for her actions.
Team Arrow wanted to save their comrade so they take the fight to The League of Shadows. As cool as it was I wouldn't have Oliver fight Ras. Cause it's Ras. That would be stupid. So if he's going to take Ras down then he'd have to be smart about it. Not going to get into full details but I see the final fight involve Team Arrow vs The League of Shadows with The Suicide Squad helping out Team Arrow. Waller wanted Ras and Oliver wants to save Sara and this is the one enemy mine that would make sense. Especially since he knows Waller would be more than willing to tear the city apart to take down Ras. And The Suicide Squad exist. The way I see it, if Superman is going to be used in Supergirl then not using Suicide Squad would be hypocritical.
Note: Why Atom isn't mentioned? Because I felt he;d be better suited as a Flash supporting character.
After the crazyness of Season 3 I'd give Season 4 a one year timeskip. Around this time, Thea has started her own superhero career and Sara has moved on to form Legends of Tomorrow. As her own way to start a new chapter after ridding herself from her past as an assassin. Her and Roy made a pretty good team honestly. Laurel continues her campaign in cleaning up Star City and is eyeing the mayor candidacy. While Oliver focus his attention in running the Queen Company. Especially now that things have quieted down since Season 3. But he has Felicity to relay him intel from Team Arrow just incase.
There is more but this is just a sample of what I would have done.
