It's no secret that the MCU is gearing up for the Dark Avengers, but it's the Justice League that could demonstrate the art of the villain team-up more successfully. The DCEU famously - or infamously, depending on who one asks - chose to veer away from the winning formula that made the MCU so successful by jumping straight into Justice League without pre-establishing half of its roster. The result of this was a film that many maligned as mediocre at best. Now, however, with the canon up in the air and a DCEU reboot looming on the horizon, this can be rectified by focusing more on the franchise's villains.
The DCEU is in a state of flux. After the success of the Justice League Snyder Cut, it's obvious that the appetite is still there for it to continue, but some things would need changing. It's obvious from Justice League that the intention was building towards an eventual clash with the likes of DC's biggest bad, Darkseid. But it's worth mentioning that one threat the DCEU could have seen before him - and could likely still see carried out with resounding success - would be the Legion of Doom.
The DCEU has one notable trait that Marvel absolutely lacks - villains. Although the merits of individual antagonists are debatable, what isn't up for debate is that as far as their cinematic universes go, the DCEU simply has more options because MCU villains tend to get themselves killed. If they don't die, it's usually because they're popular - like Loki - or sympathetic and non-villainous enough to spare, such as Ghost in Ant-Man and the Wasp. The DCEU, however, has a better track record of letting villains live, as evidenced by Shazam and Aquaman, implied by the mere existence of The Suicide Squad, and epitomized in Wonder Woman 1984. This means that, as a point of difference from its major genre rival, the DCEU can really stand out by emphasizing its varied villains in a team-up project focused on the likes of the Legion of Doom.
The DCEU already clearly had plans to implement some sort of villainous alliance, as far back as the theatrical cut of Justice League, and likely still does. When all was said and done, Lex Luthor escaped from prison and set up for a collaboration with the likes of Deathstroke. This was all the way back before Avengers: Infinity War. Unfortunately for the DCEU, Valentina Allegra de Fontaine would suggest that Marvel was ready to pick up the ball after their rival's fumble. Luckily, that doesn't mean that the DCEU can't continue to pursue this course, and potentially do it better.
Despite the recent success of the Spider-Man: No Way Home villain team-up, Marvel's villain track record is mixed. Not only do most Marvel villains die, but it's a common criticism that the weakest part of many Marvel movies is the villain, with notable exceptions like Black Panther and Shang-Chi and the Legend of the Ten Rings being few and far between. As a result, the prospect of a future Marvel villain team-up project is notably less exciting than the DCEU alternative. This is equally true for the Dark Avengers, as the two seemingly confirmed members of the group aren't outright villains or even antagonists. They're just darker mirrors of their more traditionally heroic Avengers counterparts, rather than characters who'd just as easily fill out the roster of the Masters of Evil.
The DCEU could easily forge a path to the superior villain team-up to clash with the Justice League, provided the time is taken to do so. They had the idea first. They have more options to choose from since many more of their actual villainous antagonists live to fight another day. They clearly have a minor advantage as Marvel lags behind in the villain department. Still, if they take their time, the DCEU could easily make the Legion of Doom something they can hold over their competitor.
