Then who would you suggest as the villain
The Rogues as a group.
Capt. Cold
Heatwave
Mirror Master
Weather Wizard
Capt. Boomerang (he's on leave from the Squad or whatever idea they can come up with)
Leave it at that. Let's be honest... None are prime candidates to be able to shoulder the weight of being the sole villain in film on their own. Cold would have to be the most charismatic in terms of writing and casting as he is the defacto leader. There is a wide variety of powers and broad personalities for our hero to go up against but also they don't really lend themselves to an "end of the world" story which is fine. Make it about Central City and Barry's life. Again... the conflicts and inherent drama of a young man with super powers that's taken it upon himself to help people and fight crime in his own town.
I think the template is what Raimi did with the first two Spider-Man films only Barry isn't the sad sack Raimi's Parker was, but that tone, balancing humor and heroic old school daring do is what they should go for. Plus the number and varieties of powers and skills the Rogues as a group can throw at Barry allows the film makers to showcase what the Flash can do without having to resort to another speedster to thwart Barry's own incredible powers.
Again... I think it's easier to write from my POV than trying to make a convoluted story of why the fastest man alive has problems fighting off one guy with a single gimmick for a whole movie. If it's just Cold for example I think the audience will just go "how is this challenge to a guy we know can catch bullets?"
Make it a determined and experienced crew going up Flash and the odds seem less in Flash's favor I think. Look I love the Rogues BECAUSE they aren't the Bat villains. They mostly aren't psychologically deep. You could do what they did in SPIDER-MAN HOMECOMING, sure, and basically rebuild them from the ground up, like with Vulture, but I don't know... Again the single gimmick thing is an issue for the modern audience I think.
For me if I were plotting this thing out I would hit these elements, which are more than enough for one film:
1. Thread Barry's backstory (mother's death, father imprisioned, gaining powers) via flashbacks throughout the movie.
2. Establish the Rogues as independent operators Cold brings together after each has had run ins with Flash and they attempt to put an end to his interference in their goals which includes consolidation of underworld power in Central City.
3. The problems Barry has as someone balancing beinga novice CSI, a super hero, romancing Iris and working on his father's case.
I would keep the father stuff unresolved in this film but hint at RF and maybe also hold off on Iris and Barry immediately becoming a couple.