The now ending DC Extended Universe had several planned developments and paths, though a lot of these didn't come to pass. The haphazard changes to the shared movie universe were many times based around the reception of the movies themselves. 2023's The Flash is the most obvious example of delays and changes, with the movie coming out almost a decade after being announced. That movie was also once set to have a much different villain, and this could have changed the DCEU forever.
The Reverse-Flash was set to not only face the Scarlet Speedster, but also be involved with various other happenings across the DC Extended Universe. While this sounds like an interesting development, it might have run the risk of taking away agency (and legitimacy) from the movies and their characters. Likewise, it would have also retreaded a villain who had already become rather overused in media outside the comics.
Reverse-Flash Was Planned to Appear in the DC Extended Universe
According to storyboard artist Jay OIiva, there were several abandoned ideas made along The Flash's long road to a release date. Instead of the movie's final premise -- which was based on the Flashpoint event from the comics -- The Flash would have been one movie in a DCEU subseries. The first of these films featured Eobard Thawne, aka, Professor Zoom, the Reverse-Flash, though only in a limited capacity. Reverse-Flash was going to be a villain pulling strings behind the scenes, somewhat similar to the powerful Thanos in the Marvel Cinematic Universe.
This matches the character's actions in the comics and other media, with Barry Allen's modern "dead mom" origin story being entirely due to Reverse-Flash's actions. According to Oliva, Thawne also had a hand in other events, with these villainous machinations plaguing the universe as a whole. These plans were of course dropped, as was the idea of Zack Snyder's Justice League trilogy being followed up by a more overt Flashpoint adaptation. Thawne himself didn't show up on-screen in the DC Extended Universe, with the closest thing to a Reverse-Flash being a version of Barry Allen who becomes Dark Flash. This all sounds like one of the DCEU's many missed opportunities, but it was ultimately for the best.
Reverse-Flash's Planned DCEU Appearance Would've Admitted Defeat
The DC Extended Universe was incredibly controversial, namely due to how it handled characters like Superman. Things crescendoed with the polarizing reception of Batman v Superman: Dawn of Justice, which quickly cemented the shared universe as negatively received and "too dark." Some fans called for a reboot, and as soon as it was suggested that The Flash would adapt elements of Flashpoint, this only intensified. Confusing directions and last-minute changes definitely affected the films, with numerous post-credits teases doing little to advance the universe's story. Though the exact plans and manifestations for Reverse-Flash's inclusion are mostly unknown, it's likely that he would have represented these criticisms.
In other words, Thawne's actions having ripple effects across the DCEU would serve as admission that the shared universe was in a "wrong" place. In a sense, it would be saying that these divergences from what fans expected were simply the result of a villain, and that by defeating him, Barry Allen could restore the "real" movie universe that fans wanted. It's essentially what's being done with the upcoming DC Universe reboot, though that's at least far less cynical. These new films are wholly removed from the previous continuity, so while some fans may see them as "finally getting things right," they're seemingly not being made in such a dismissive manner to what came before.
It's possible that Zack Snyder and the initial directors behind The Flash didn't quite intend for him to be handled that way, though that's certainly how these concepts sound. It speaks to how mismanaged the DCEU as a whole became, with the divergence from Snyder's ideas only compounding the issue. Despite some of the films afterward being well-received and even highly profitable, it's also true that the universe became a series of sometimes incongruent ideas that didn't fit with what came before. Thus, even if the execution was better than the idea itself sounded, Reverse-Flash's being a mastermind behind the DC Extended Universe was probably going to fall through the cracks anyway.
Reverse-Flash Didn't Make Sense as the DCEU's Ultimate Villain
The biggest problem with the idea of Reverse-Flash being the most paramount villain behind the DC Extended Universe is that it simply doesn't fit the character. His vendetta is against Barry Allen, and while this might sometimes be extended to other Flashes such as Wally West, it's almost always contained to the Flash Family and Barry's legacy. An example of this was when he masqueraded as his rival in the story arc "The Return of Barry Allen," which was also a huge point of development for Wally. Making him integral to the lives of other DC heroes feels too far removed from his usual aims. The main thing that keeps Reverse-Flash's immense power in check is his incredibly petty attitude, as he'd be so much more if it weren't for his overriding hatred of The Flash.
This role was far more fitting for someone like Darkseid, who was clearly part of Zack Snyder's actual DCEU plans. While a proper Flash trilogy with Reverse-Flash as its ultimate villain might have worked, making Reverse-Flash important to any outside movies might have downplayed Darkseid and foes while making Thawne's ambitions too big. That's not even getting into the fact that Reverse-Flash is already used enough as is. He's the only villain of Barry Allen's that has any sort of close relationship with him, as the more nuanced takes on the Rogues was developed during Wally West's tenure as The Flash. Reverse-Flash was constantly brought back in the Arrowverse Flash TV show, despite some of these returns being outright illogical. With how overdone the villain had already become by this point (and the show itself ending right as the movie finally released), it's best if Eobard Thawne remained off screens (both big and small) for the long haul.
The Flash is now streaming on Max.