Warning: slight spoilers for Justice League are in play. If you're not current, bookmark this page and come back once you've seen the film.
Ask any superhero, and they'll tell you that part of doing the job right is having the correct attire to match the form and function of your abilities. Of course, trying to come up with the right threads is something that takes a bit of time, as you can see in Barry Allen's Flash suit in Justice League. That's part of the individual charm of his battle gear, as costume designer Michael Wilkinson explained how Allen's unique look sets up his character, as follows:
He's a very unusual combination because he has all of these high-tech elements, but he's basically just a kid. He's just starting to come to terms with his powers. He doesn't have a lot of money. He hasn't been given a suit by someone else and he's kind of making it up as he goes. [His suit has] this great quality of almost a prototype suit, where he's working through these ideas. Some of it works; some of it doesn't. It's all scratched and busted up; he has these high-tech materials fused with this grassroots, skater punk aesthetic.
Wilkinson's commentary on just how thrown together The Flash's suit is almost makes the scene where Ben Affleck's Bruce Wayne is giving the suit worn by Ezra Miller the once over that much funnier. In said scene, Batman's billionaire alter-ego highlights the pieces of the suit that actually function. But, as Michael Wilkinson's interview with Fashionista goes to show, it's not exactly the final form that Barry will need in order to achieve something as fantastic as what he'll eventually have to do to set off the events of Flashpoint.
This can even be seen if you go back to both the "dream" sequence that Bruce Wayne has in Batman v. Superman: Dawn of Justice, as well as Barry's surprise cameo in Suicide Squad, as both incarnations have slightly different looking costumes to the one that we see in Justice League. In the case of Barry's future self, it could be stated that he eventually gets it right. And his Suicide Squad outfit is another iteration that Mr. Allen went through in his quest to develop a working super suit. So while most other heroes have had a static appearance throughout their DCEU appearances, Barry Allen's Flash is going to have some more changes ahead.
This opens the exciting possibility that in future appearances, Ezra Miller's costume could change with the future of the DC Extended Universe. At least in as many appearances he makes before Flashpoint solidifies his look from Batman v. Superman: Dawn of Justice. And much like his costume, we'll get to see Barry Allen's personality change after experiencing the events of Justice League, as well as whatever the fall out of his post credit race with Superman.