Feel free to disagree with me, because I am of course forbidden by national referendum from writing TV shows. But this is what I think...
1) Mirror Master.
But 46 episodes (plus crossovers) and not a single sign of the Mirror Master? What the hell?
So there's plenty of room for classic rogues. So why no Mirror Master?
Mirror Master and his relatively low-tech powers seem like a perfect match for the "we can do it cheaper than CBS" CW show.
2) Prune the cast, and let Barry be a scientist again.
Most Berlanti shows are a bit overstuffed in their casts. It's easier to have lots of people stand around talking than actually having stuff happen.
But the Flash really bursts the seams. There are too many cast members!!
And the worst part is, too many of them are scientists. And far better scientists than Barry Allen.
The opening monologue (again, a requirement for any Berlanti show, because the audience is too dim to pick up on the premise of a super-hero show without a weekly reminder of that premise) tells us that Barry is a "scientist."
The problem is, there are always at least three other scientists on the show at all times, and they're ALL smarter than Barry. Apparently, setting the HQ at S*T*A*R Labs means the show is contractually obligated to have at least three scientists in the control room at all times, talking in Barry's ear because he's not smart enough to figure out the tiniest problem without their guidance.
Amongst the brainiacs manning the control room this season were Cisco, Caitlin, Harrison Wells-2, Jesse Wells, Martin Stein, Felicity Smoak, Jay Garrick (well, whom we thought was Jay Garrick, and it is fascinating how a deranged serial killer was suddenly smart enough to be a super-scientist, but hey, comics), Christina McGee...I'm sure I missed a few. Barry even had to time travel back to last season to consult Harrison Wells-1/Eobard Thawne on some science.
And so every time--every single damned episode--scientist Barry Allen is completely baffled by the latest villain's powers/scheme until the brain trust chimes in on his earpiece something he really should have thought of himself. And because we have to service the needs of this immense cast of scientists we've put on the show, one or more of them have to be the one to solve the problem while Barry runs around futilely, and we end up with our star being probably the 5th or 6th smartest character on his own show. He's lost a lot of his agency as a lead character.
Contrast this with Barry's guest spot on Supergirl. There was no one to talk into his earpiece. Suddenly, he was the smartest guy in the room, he was getting to do the scientific lectures and build things and solve problems. He was suddenly brash and confident, things he's not allowed to be on the Flash because then all those actors you've hired would have nothing to do every week.
[It should be noted that the Supergirl show suffers from this, as well. There's really no reason Kara Zor-El should need someone in her ear to talk her through a fight, or need to have a federal SWAT team accompany her on every mission...]
So get rid of S*T*A*R labs (how do they even afford to keep the lights on there, anyway? Do they actually do anything, produce anything, sell anything?). Send a couple of our boffins on extended vacations, or send them off to Star City. Focus more on Barry as forensic scientist. Let him solve problems himself.
Let Barry Allen be Barry Allen.
3) Mirror Master.
4) Enough with the time travel already.
Look, it's a TV show. It's a super-hero TV show. So we know there's going to be some scientific illiteracy and downright foolishness on hand.
But these writers have no idea what they're doing with time travel. They have no idea what their rules are, even from episode to episode. And it's become worse than Christopher Reeve turning the globe backwards because there was a bad outcome, because The Flash does it again. And again. And again.
They keep changing the premise--changing the time line will cause bad things. Except when it won't. But then it does again. Changing his past makes Reverse Flash never have existed--except everything he did and said still happened. Oh, and then it didn't wipe him out of time, anyway. Because.
It's also become a fairly reprehensible deus ex machina, as at least three times in season two, including the climax of the whole season, we've seen characters killed only to be told later that it was a "time remnant," which basically means you let a "younger" version of yourself die in your place while you continue merrily along with no ill effects (and apparently not changing the timeline because...?)
It's lazy, terrible storytelling. And we're stuck with it, because of what happened in the season finale--the season 3 opener is allegedly named "Flashpoint." But for heavens' sake, let that be an end to it.
Also, nothing in the world is more offensive than continually saying that running at Mach 2 is enough to break the time barrier. That's so stupid, even DC Science mocks it as being unbelievable.
5) Abra Kadabra
[Extra bonus points if you can get Penn and/or Teller to play Abra Kadabra]
6) Good lord, enough with the brooding already
It is not at all fair for me to say, "Good god, Barry Allen, get over your dead mother already!!"
But then again, the show itself has done that already, dedicating an entire episode to the Speed Force itself telling Barry that the only way he'll be the best hero he can be is to "let her go."
I'd call that a nice bit of self-aware meta-commentary there. Except just 2 episodes later, Barry gets all melancholy again, and goes back in time (again) to save her (again). Apparently the show's creators don't pay any attention to what they've just written 2 weeks ago...
Obviously, it's a diktat from Geoff Johns (and DiDio, and Jim Lee), that heroes must have tragedy in their history in order to be heroes. And sure, that works fine for Peter Parker and Bruce Wayne. But must every hero be motivated by tragedy? Aren't most real-life heroes motivated by, well, just wanting to do the right thing? Not to Johns, who retconned Flash to have his mother murdered and his father framed for it, and for Hal Jordan to watch his father die in a plane crash. Yay, super-heroes are fun!
Anyway, The Flash show obviously adopted that mantra, to the nth degree. Even Batman doesn't genuflect to his dead parents this much! If Bruce Wayne were to watch this show, he'd say "Jesus, Barry, get over it!" To have Barry this mopey, and continually wallow in his family tragedy, does not make for a better hero or a more entertaining show.
Check out this fact: Nora Allen, as played by Michelle Harrison, has appeared in 11 of the episodes so far. Despite being dead, she's been in nearly 1/4 of the episodes broadcast! Flashbacks, time travel, time travel again, Speed Force ghost, time travel yet again, yada yada.
Even if you buy the necessity for the tragedy of his mother to motivate Barry Allen, he should have other motivations, shouldn't he? And you shouldn't constantly bring it up again and again every week, should you?
Find some other stories to tell.
7) Mirror Master
8) Enough with rival speedsters as the Big Bad
I mean, come on now, guys. Exercise that imagination a little more.
9) Enough with the murdering and incarcerating villains
Last season, Wells-2 straight-up murdered Turtle in cold blood. Turtle was locked in a cell, powerless, and Wells killed him to take a sample from his brain. And the only consequence, literally, was that his daughter was mad at him for awhile. That is seriously f&*^%d up.
That's what happens when you can detain defeated crooks with no due process, locking them in an ersatz prison with no trial, no judge, no jury, no rights. There's nothing to stop these guys from taking the next step to executioner, and obviously some of our cast have no qualms about taking that step. And even worse, our hero apparently doesn't have anything whatsoever to say about the issue, except to shrug--another consequence of not being as smart as his supporting cast, and having little agency.
Build an Iron Heights set, CW. And let's punish murderers, OK?
10) I bet you thought I was going to bring up Mirror Master again, right?