So, Late to the party here, but for some reason I've been thinking about this a lot, and put way too much thought into this. So enjoy reading my armchair writing here, or don't, I just needed to get this out.
In the aftermath of a Nuclear attack by not-Russia-but-still-kind-of-Russia, tensions between USA and Russia are at an all time high. Like, Cuban Missile Crisis high. Lyla and ARGUS was able to keep a war from breaking out (and the whole stolen by a terrorist thing), but it basically kicked the world back into the worst point of the Cold War.
Oliver and Felicity are covering the crimes of Star City by themselves, because Felicity is kind of a narcissist and unable to cope with not being in the action, whereas Oliver is in a really bad emotional state and kinda can't be alone right now. It's unusually quiet and Felicity isn't happy because it was starting to remind her of her boring home life... until Captain Boomerang comes back, pissed off being locked away in hell for 2 years, he's come back to be... someone else and starts wanting to murder the Green Arrow for putting him there. Due to being completely understaffed, it takes GA longer to deal with the Captain than would be expected (like, 2 episodes) - mostly for Ollie to realize he can't do it alone. Oliver also needs to cope with the fact he murdered the crap out of Darhk, which while apparently okay in the public eye, he is going through hell over because His promises to Tommy and Laurel made him want to never do so. Ray returns to Star City and is given back his company (Which is on its last legs after Felicity was a terrible CEO, so might as well try anything), and he starts a new section within Palmer-Tech: Q-Core, ran by Curtis Holt. Holt now gets to provide Oliver with some Utility Belt-Level Arrows, so we have a different way of saving the day then Felicity finds some way to hack it somehow. Lyla is busy with International affairs and keeping the world safe, while Diggle is off wherever, so Felicity gets to impart her pseudo-scoiopathic ways to baby Sara - which makes her want one of her own (WITH OLIBBUR), Unfortunately, After Oliver watches Black Siren die, he realizes his Laurel was wrong, and that his true love isn't Felicity, and they can't be together. We still need some CW-level stupid drama y'know. Felicity's dad comes back and wants to properly reconnect and atone - even helping GA if he can, but must do battle with Donna and her mindset of "PEOPLE CAN'T CHANGE YOU MONSTER". He can change or not, but not until like several episodes later.
Mayor-Oliver has issues when the failing GA is unable to keep Star City safe, and leads to the people thinking GA can't keep them safe and declare more help. He faces a ruthless politician trying to remove Pro-Mask Oliver Queen and crack down on the ever-rising vigilante problem. This will lead to a 176th time there'll be an arrest warrant for GA. Lance comes back (after breaking up with Donna when he learns Donna lied MAJORLY about Felicity's backstory yet she was so pissed off about a small lie for Lance to reclaim his dream job and help people, or not Donna can come and be terrible again, why not) Allies with Oliver and represents a public face for the vigilantes appearing across the country.
Only the calm before the storm though, as Sharp (Girl who stole Black Canary stuff) makes an appearance and quickly takes over the Star City underworld, with a new modified version of the canary's collar, although without certain sonic-qualities. She's the new Vertigo, is what I'm trying to get across. Also not a villain of the week, she's kind of a big baddie. And also appears in a halloween epsiode where she dresses like comic book canary and is like "Who fights crime in fishnets? AWESOME PEOPLE THATS WHO"
Thea went to Coast City (or somewhere), because reasons, and she's inadvertently making headlines from not being able to hold back her superhero-isms (gonna save a couple people from burning buildings, stop some muggings, etc...) but still having trouble dealing with her wanting to stop being a hero. Anarky is going to hunt her down (repeatedly), and through a wide number of cringe-worthy comments, turn her evil. I dunno what villain specifically, maybe Cheshire, or the real Dark Archer. She finds some issue with Oliver thats been building up for the last 10 years and it boils over to the point she wants him to suffer. She doesn't try any destroy the city crap, nah she just wants to hurt her brother.
Flashbacks are gonna deal with Ollie going to russia and almost causing WWIII again with some nuclear bombs and whatnot or something, whereas his escapades accidentally screw over Anatoli, whichc puts him in a lot of trouble, and framed by some 1%-er plot, turning one of his closest friends against him and reminding Oliver of his vendetta from his Dad. Anatoli blames Oliver for this, and comes back for season 5 trying to abuse the high tensions and actually cause WWIII as the KGBeast. More flashbacks about Thea grieving over Oliver and Laurel helping her cope through it - illustrating how terrible it is for Thea now that Laurel died.
Also an episode where GA decides he needs to do right by his biggest enemies, namely Slade. to do so he brings Shado's random twin from a flashback last year to hear Slade's story of her sister... stuff happens they fall in love Slade leaves happy and decides to break his promise this one time.
Basically, nobody cared about the nuclear bomb and Laurel dying and whatnot because they could 100% focus on Darkh, but now theres nothing special going on, the trauma is finally starting to affect them.
Gonna get a crossover episode where Black Siren escapes and casues havoc in STAR City. GA can't deal with her and calls in the Flash. The Flash in a weird different BLUE FLASH WHO STRAIGHT UP MURDERS BLACK SIREN and causes a lot of problems between Barry and Oliver. slightly different Flash, thats Flash storyline tho. No Supergirl yet, shes gonna show up on Flash first. Also I can't think of something for her to do yet. anyway, Arrow goes to Team Flash and helps the team cope with the fact that something is up with Barry.
Another crossover where Sara comes back pissed off at Oliver because She saw Flash murder Black Siren and OLIVER DID NOTHING. They scuffle and reconcile, and Sara joins the team again for a little bit.Can't stay too long or the Oliciters will kill her though.
Like, Mid-season, we get a visit from Nightwing?, Robin? NOPE, Tim Drake? Bluebird? some not batman part of the bat-family. They've come to star city, becuase the recent recurrence of Vertigo has a special extra feature, some components similar to the Fear Toxin of the Scarecrow! GA and friends eventually save the day, and GA shares a certain "Proposition" with our new Bat-friend. Some cool set-up for the upcoming JSA - including them coming back near the end of the season to help keep STAR City from calling apart.
Diggle decides to abandon his life of crime-fighting but can't quite cope with it. Part of it includes leaving his wife and child while he does soul-searching. gonna become a hippie and travel in a truck cross country with some hippie, and when he gets back hes gonna spread his newfound liberal ideals with Ollie. Or something, I don't really know what Diggle will be up to.
Anarky becomes more influential, appearing to be the reason behind the corruption of Thea, New BC, and leads to taking over the city, while BC is on her own stuff, and Thea is busy trying to torment Oliver. Oliver is distracted with Thea and the city is falling apart under new Vertigo and Anarky, so Oliver calls in some help, basically resulting in not-officially forming the JSA. By the end of the season, Oliver announces the official creation of the JSA.
Basically, Ollie has like 4 Big Baddies this season - KGBeast, Vertigo, Whatever Thea will be, Anarky kinda and I dunno, throw in Merlyn or something. No Voldemort or anything, just people with personal vendettas and the city will kind of just go down with Oliver.