1. Barry Allen needs to be the focus of the film. Period. His character needs to be defined in a way that that is more than just broad strokes as "geeky nice guy with goofy sense of humor." Barry should be a person that embraces science and rationality even though he lives in a fantasy world where the irrational is in more ways than one is becoming more common place.
That said... I kinda hope they ditch the whole death of his mother angle. I think it would be amazing if his morality and heroism were actually a tribute to his two LIVING parents. Plus, given the way that death is such a key part of the show it could accomplish two things: 1. It differentiates the show and film and 2. You could get some dramatic tension if the finale is in fact a threat against the mother/father in some way. People familiar with the show would assume they would die. Also, it's just me but I think a super hero who does what he does not because the "villain" made him, not because their origin is linked, not because he suffered a tragedy but simple because he truly believes in what is just and good, would be a breath of fresh air. And I say this as someone that LOVES MOS and similar type super hero stories to pieces.
Barry also needs to be shown as more self sufficient as a super hero (he's supposed to be a really good criminologist and forensic CSI... SHOW THAT DOES'NT DISAPPEAR WHEN THE RED SUIT GOES ON!) but far more out of his depth in his day to day personal life, thus necessitating the relationship with Iris. To ground him, to help him grow, to give him solace... BUT NOT TO BE THE KEY TO HIS SUCCESS AS A HERO BY WHISPERING THE POWER OF POSITIVE THINKING IN HIS EARPIECE.
2. Iris needs development too. Show what makes her so appealing to Barry in the first place. I say instead of a sudden career shift make Iris someone that is a dedicated muck raking reporter, or at least show that's the trajectory she's been on for a long time post College. Make that a key connection. Iris' journalism and Barry's science, both are about cleaving truth from falsehood in a way. I would make it this has always been their connection, with the addition of great compassion thrown into the mix. Truth and compassion are pretty solid grounds to build a relationship on.
Which is another thing... I would have them on a break. They were a long term couple but after Barry got his powers he's pushed her away for fear of what could happen to her. This would be about a couple reconnecting, not instant love. It grounds a relationship that we only have a small part of a 2 hour film to focus on. Iris isn't whiny and pining for Barry, but she is truly hurt and baffled by how he seems to want her still but is keeping her at arms length. I would even have that the Flash is a story she is pursuing and she alone uncovers the truth. It would give her credibility to the audience. Finally... absolutely NO damsel in distress moments in the first solo film. Not a damn single one. Keep Iris with the exception of reporting on the Flash separate from the schemes and danger of the villain or villains. Save that for a sequel.
3.I would have Wally as a supporting character but make him a real pre-teen kid. Not a 19-20something heart throb. Wally would be, as he always should be, the Flash's number one fan. Show that his relationship with Barry is like a blood relative even though we all forget... They aren't. But that's what it should be. Have Wally be this positive, if naive presence in Barry's life. Save the power stuff for another film. For now Wally personifies the public's reaction to the Flash and his heroism. But I would also make him an important part of Barry's life away from the costume. He's a smart kid that Barry enjoys having a positive impact on. Wally loves Barry in the way of that younger uncle/nephew thing. Barry is old enough to be an authority figure but young enough to relate to a kid in a way other adults Wally knows cannot.
4.I think that for a first solo film you shouldn't have another speedster enemy. Save that for a sequel. So table the Zooms/Revers-Flashes for now. Concentrate on the the Rogues, or insert someone like a Vandal Savage into the mix. Perhaps as the mastermind behind the Rogues. Define them as pros-pros type meta-villains. This ain't the badguys from Gotham. They are here to get paid, at least most of them. This in a way makes them more dangerous than just freaks with a gimmick. They are all intelligent and sanguine criminals. Think Deniro in HEAT. Not a mad dog killer but if they gotta get out that door and you're standing in their way...
Snart of course is the top dog, Heatwave his compadre. Then there's Weather Wizard and Mirror Master. Combined, for a first film that's a rough crew for a rookie Flash to handle. They should be charismatic but excepting Heatwave they should all act in a ruthless if sane manner. Make it an alliance of convenience. But also add in the blindspot of professional pride. These guys can't have their reps ruined by a jerk in a red suit who makes corny jokes. If Flash wasn't around their heists and other jobs would be legendary. Their careers before Flash showed up were just that. Maybe have that as a subplot that becomes the finale? Barry tangles with each, defeating them easily throughout the film only for them to join together for the big finish, taxing even the fastest man alive.
I could see the Rogues alone but I think a villain like Vandal Savage would be good opponent for Flash as well. His immortal nature could be a way to link up past, present and future if the finale were to include Flash breaking the time barrier in some way. Could also be a way to bring in the 25th Century Reverse-Flash and Abracadabra. You could use the old saw of self fulfilling time travel to explain things, such as Flash going into Savage's past which sets events in motion only for something Flash gets or learns in the past also be the key to defeating Savage and thus, with no money coming their way, drain any immediate motivation for the Rogues. Just an idea.
5. Barry needs to be light hearted but the drama and the "fun" have to be balanced. Maybe make it about balancing out how amazing and fun it is to be the Flash and do what he does, with the real issues that would come up in such a life and play those issues straight. The isolation of keeping a secret. The feeling of loss of a woman you think you can never be with even though she feels as you do. That stuff can strike a cord. Of course Flash by his nature should be a corny quipper and the set ups for funny stuff is built in. But that's why I think they shouldn't have to turn up the lightness to 11. Keep both the seriousness and fun about even.
