What Are Some Things You Would Like to See Added Next Season

Arrow - Kill off Felicity
Gotham - Change actors for Bruce and Selina
The Flash - Kill off DeVoe and have a serious villain that is a physical challenge for Barry
Legends - Add Nyssa to the team
Supergirl - More Saturn Girl less clothes

More cats. The cameo by Streaky in a dream sequence isn't enough.

Arrow - Kill Renee, break-up Felicity and Oliver.  Explore the neglected character aspect of Oliver being a philanderer; the key difference between Oliver Queen and Bruce Wayne has been that Ollie really is the playboy Bruce pretends to be.  I have to admit, I kind of miss the flashbacks, as meandering and useless as they were the last couple of years; done the right way, I'd like to see them find a way to bring them back.  I think a good one would be Oliver's road trip with Hal Jordan, which I realize may be logistically impossible, but I'd still like to see it.  I've always thought Michael Shanks could make a fair Hal.  I think Oliver meeting a new reincarnation of Hawkman could be good too; and explore their polarizing relationship.  As far back as that dynamic goes back between the two of them, it's particularly timely now; though I think they'd need to make sure not to make their disagreements lopsided, where one is always right and the other always wrong, but find a way to show the merit to at least some of each other's opposing opinions.
Smaller core team, and introduce more characters operating on the outside, who don't know Oliver's identity.  It would be interesting if Oliver and Carter, or whatever name he might go by as a new incarnation, interacted in their civilian and masked identities, but just never find out who the other is; probably because they don't get along so well as their alter-egos.  But the show definitely lacks the grounded perspective it used to have.  
And as enshrined as she is as a main character on Legends, I'd bring Sara back to join and lead Team Arrow; becoming the definitive Black Canary and eventually get together with Oliver.
Arrow might benefit most from what Agents of SHIELD has been doing, dividing their seasons into two or three mini-arcs that feed into each other.  Oliver sent to Supermax, Oliver on the Road, Oliver returning to Star City...

Gotham - Don't watch, don't really have a horse in that race.  With that said, for a while now I've thought it would be an interesting move if they killed off Bruce; and firmly established this as a complete departure from all expectations and any established mythos.  It's a world without Batman, where there will never be a Batman, yet everything else, all of the villains he would otherwise face and defeat remain; and the likes of Jim Gordon and the other protagonists have to find another way forward.

Supergirl - Power Girl would be good; Helena Wayne's Huntress could be another addition, friend and ally to Kara, much as PG and Huntress were in the New 52.  A reformed Banshee who becomes a friend to Kara in her human persona, plagued by the Banshee curse could be another story.  I think less focus on Kara's personal drama would be nice, and have her do more investigative reporting; uncovering short run mysteries could be good.  
......Lena and Kara confess their true feelings for each other.....?

Flash - Adding a touch of police procedural format wouldn't be the end of the world; show Barry actually working using his forensic analysis skills, working cases that only last one or two episode, slowly adding in and building the season long arc that later takes central focus towards the end of the season.  
Allow Cisco, Caitlin and Harry to occasionally leave the lab and conduct independent lives from one another.  Find a way to clear "Harrison Wells" so Harry can just be himself on this world.
Add a new "Captain Cold" and "Heatwave" - probably with a tense encounter between the new Heatwave and Mick; allowing the Flash to have it's major Rogues, giving Legends the freedom to grow Mick into whatever type of character they want him to be.  Maybe the new Heatwave comes about as a result of Mick's time travel, where Mick is never born, or doesn't lead the life he originally had (especially after meeting his dad); and Mick becomes a different type of person and someone else takes up his mantle as a firebug.

Legends - It seems like it would be too late now to establish better rules for time travel, but this is as much a wish list as it is anything else; so I say the Legends, and perhaps the rest of the Arrowverse, find themselves at Vanishing Point, where they discover some type of paradox machine, which is disrupting the normal function of time, which is supposed to be immutable.  They destroy the machine, which has some radical effects on everyone at ground zero in the short term - experiencing some wild alternate realities, as time resettles to what it's supposed to be.  The machine would be found to have the specific design of removing someone, or several someones, from history; which could somehow be a way of introducing the original comic book version of Wally West, or better still, the "Holy Trinity" (Batman, Superman and Wonder Woman) - which could be an in-story explanation for why we've never seen these characters until now, because history has been changed to prevent them from coming into being, at least as superheroes.  From that point forward the past is "fixed," but people can still travel back and they end up part of those events.  The Legends are no longer trying to "fix" anything - which they sucked at anyway - but just generally being heroes, helping people in time who need their help.  More stories set in the future, maybe explore the multiverse.  Introduce Booster Gold to the team, maybe Ralph and Sue; and have Rip be a more permanent presence on the show again.

That would be awesome!