Before I get into this in any amount of depth let me say who the Flash must be for a movie, Barry Allen.
1. Before we are introduced to Barry Allen, we are introduced to Johnny Quick. Johnny Quick is a speedster who acheives his speed by speaking a math equation (("3X2(9YZ)4A") In the movie Quick needs to be the first person to acheive these speeds. He is only able to run quickly for a short period of time after the formula, meaning he needs to say it multiple times.
2. This formula MUST replace the speed force. The speed force in the simplest explanation, is a force- that can somehow have people travel into it, and get lost there for years- that gives the Speedesters their speed abilities.
3. Barry's terrible accident must be different. Barry must discover this formula for himself, and upon saying it for the first time he must spill some kind of chemical then. No combination of gasses and a lightning bold, the formula must be used. And this chemical some how made the effects of the formula permanent.
4. NO COSTUME IN THE RING. Flash is the fastest man alive, if Barry really needs to change very quickly into the flash, speed home at the speed of light, and change. It will make him look like he is gone for a fraction of a second.
5. Iris West- technology addict. Barry Allen and Iris West must already be lovers when Barry has his accident. The Flash can't just know where trouble is, he needs his wife, Iris- the woman who is always coincidentally at the scene of all of the crimes- to tell him. "There is trouble at 31st Street." And then he will instantly be there.
6. Show Barry Allen as a regular person. He is not going to run everywhere he needs to go, he is going to drive. He is going to get stuck in traffic, and hate it even more than everyone else does. His speed must bother him. He will be zooming past everything in his world. He is someone tha can finish all of the things he needs to do in a day in 30 seconds. But he has to drag out his work day, 8 hours feels like 10 years to him.
7.This might be simple, and goes without saying but whatever- NO KID FLASH. sorry Wally, I'm a huge fan of yours, but keep your role in the movie to the minimum, just being a nephew.
8. Jealous cops. In superhero movies the cops are always huge fans of the superheroes, they help out in impossible situations for a human to deal with. The Flash though can stop every crime every day if he wanted to. Why are there cops then? They must be jealous, and fearing for their jobs.
9.Barry's mother must be dead(like she is in the comics.) The murder of his mother lead Barry to feel the strong sense of justice that drives him to be a hero. If his mother is alive, she will rely on him for many things. A son that can run at the speed of light, imagine all of the errands he could run. And if she just died of old age, Barry might not have the sense of justice.
10. Multiple villains. I know this is usually something that is an awful decision for Superhero movies, but the Flash needs it. Think about it, FLASH v. CAPTAIN COLD. The Flash could easily sneak up behind him, grab him and the movie is over. The movie needs the rogues gallery. The only way to defeat the Flash is to have multiple crimes occuring at once. Flash has the best rogue gallery, show it off.