The Big bad should be more then the last rogue standing it should be the villain who moves the plot forward.
If you are going to make MM the Big Bad, you shouldn't have the other villains make fun of his costume, unless he is going to punish them right away to maintain his menace.
Or you can spring his payback as a surprise. As long as there's appropriate payback it will be fine. Revenge is a dish best served cold is the saying, not revenge is a dish best served immediately.
And if a humorous line doesn't work ultimately, you edit it out. Just like they've been doing for 100 years.
The spider-Man movies didn't have henchmen and they did fine, building the action up for Act to act.
The Spider-Man movies had villains that were on a par with the hero physically. If not ahead of him.
Plus, it's believable that a flying villain can get away from Spider-Man. Other than MM, how does any Rogue get away from the Flash in a movie? The Flash's whole power is that he can outrun everything and everyone.
Flash's origins, Flash fights some regular crime, Flash foils one of the rogues while committing a robbery and Mirror Master saves said Rogue, Captain Cold decides they have to eliminate the Flash, so they step up a trap that pulls all the stops and uses every advantage they to make the fight tough for Flash. Flash wins, end of movie.
There's no Act 2 there. The second half of the movie is 2 action scenes and a "we've got to kill the Flash" bridge. If I'm aiming too big, you're aiming way too small.
And, it's really important to point out, Flash's origin is really short. I got hit by lightning and chemicals. I've got superspeed. Hmm, I'm a cop and a superhero fan, how can I possibly put those two things together?
Flash being less powerful then in the comics and being a rookie can explain why he is having so much trouble with them.
Or you skip the rookie stuff and give him big problems so that he looks just as exciting and competent as the heroes that are already in their 2nd and 3rd movies.
Magneto who was clearly the Big Bad, received any character focus in that movie, Mystique had one line, Toad had one line and Sabertooth had two lines.
Great, and how many lines did the bar owners that Wolverine fought get? Or the senator in scenes where he was captured by Magneto? All that time can go to fleshing out the Rogues.
Or, here's an idea, you don't have to make the big bad as dominant as Magneto and can spread some of that around to make the group more egalitarian. How many lines did they give Sir Ian McKellan, great Shakespearian actor and one of the finest actors on the planet?
Except their motive by the end of the film would be kill Flash, not just steal stuff.
Which is about as easy a transition as there is. "We've got a problem stealing stuff, The Flash. What do we do? Kill him."
C'mon, that's not at all difficult in any fashion.
With the main point, The Rogues pretty much have the same goal. They're really one character with multiple facets. One of them doesn't want to rule the world. Another doesn't want revenge. And a third doesn't want to nihilistically destroy everything. They may squabble, they may have internal politics, but they're on the same page and already organized. There's no, how do we combine plotlines transition going on, they're already on board.
But again there wasn't whole scenes where Boddicker and his gang just sat around a table and talked to each other. They had small little exchanges while doing something else, like showing off some weapons. Not mention common thugs are less expensive for a budget then super villains.
1. The Rogues aren't expensive. They're not CGI creations with fancy animatronics and makeup. They're guys in costumes. Five Rogues are cheaper and easier to film than one scene of Dr. Octopus. Heck, they're easier to film than Nolan's Two-Face. You go in, put on a costume, no more makeup than any other actor, and you're ready to go.
2. Boddicker's crew was toting around military weapons by the end of the film and blowing up all kinds of stuff, which I think is a fair comparison to what the Rogues will be doing. Also, ED 209 is an ally. How small a budget do you think a Flash movie will have?
3. There's at least one whole scene of Boddicker talking to Dick Jones. And a couple more short scenes of interaction. And all of the OCP internal politics stuff. Heck, Robocop has 3 plots going on. Dick Jones and OCP (along with a police strike), Boddicker's crime wave, and Robocop's origin. A Flash and Rogues movie is only likely to have 2 plotlines, Flash origin and Rogues crime wave. And two subplots, Iris and Rogue politics.
And, really, Flash's origin might be the single most straightforward and easy origin there is. Padding it out is likely to work as well as the silly "origin of the bowtie" stuff from Flash Rebirth.