Catwoman already had her own live-action feature... a very, very bad one that had almost nothing to do with why we love the character. The time is nigh for a serious Catwoman film, bridging the divide between hero and villain in the DCEU, and talented actors would line up around the block at the opportunity to play her.
Take Spider-Man's origin story, and have him learn the exact opposite lesson. That's the idea behind The Hood, a supervillain who rose from obscurity to become a dominant force in the Marvel Universe. It's Goodfellas with superpowers, and it would be a hell of a good movie.
When Warner Bros. cast Jesse Eisenberg as eccentric billionaire tech genius Lex Luthor, they knew that audiences would immediately think of The Social Network. So a film about Luthor's rise to power, or his return to glory after his short prison stint (and possibly a presidential campaign, going by the events the comics), is right on brand.
Marvel Studios has a lot of great superheroes but most of the supervillains haven't made much of an impression... except Loki. Tom Hiddleston is one of the MVP's of the MCU, with fans who want to believe that the God of Evil is a secretly decent guy who's eager to change. Give him a movie that toys with those emotions, and you've got yourself a hit.
If you think Superman's an outdated hero, then you've got to read Whatever Happened to Truth, Justice and the American Way, which pit the Man of Steel against a new team of heroes who represents harsher, modern ideologies. Dangerous but frustratingly topical, these "heroes" - who eventually allow themselves to do horrible things - would be a smart cautionary tale for everyone in the DCEU audience who thinks heroes need to be "dark" to be good.
The utterly bizarre Captain America villain MODOK - who is a floating giant head in a chair - assembles a team of mostly third-rate villains to pull off the crime of the century in this excellent Marvel heist caper. It's not about bad guys doing good, it's about bad guys doing bad, double-crossing each other, revealing they were completely different bad guys all along. MODOK's Eleven would be a completely unpredictable crime comedy from the MCU.
Already introduced in Spider-Man: Homecoming (he was played by Donald Glover), The Prowler is skyscraper-scaling burglar who turns to a life of crimefighting because, deep down, he's not such a bad guy. The Prowler is an opportunity for an MCU (or Sony Pictures) movie to explore different angles of the superhero experience, and dramatize the path to redemption after a life of crime.
The Riddler has been suspiciously absent from Batman movies since the 1990s, but that's due to change. Take the enigma-obsessed Edward Nygma and make him a reformed criminal, working as a private detective for other supervillains, adapting one of the cleverest and most interesting comic book storylines the character ever received. Batman's busy saving the world from aliens, so SOMEBODY's gotta be the world's greatest detective... right?
The rebooted Teenage Mutant Ninja Turtles movie franchise seems to have stalled out. Maybe the films were too big for their own good. Next time, try this angle: the tale of Oroku Saki, a ninja who rose through the ranks of the Foot Clan, vanquishing his rivals along the way, until he became not only a criminal leader but a viable conquerer of the world. Until those damned Turtles come along in the sequel, of course...
When all of the major superheroes disappeared in the Marvel Universe (it's a long story), The Thunderbolts showed up to save the day. The world loved them, but there was one problem: they were secretly supervillains, who planned to use their celebrity to make world governments drop their guard, leaving them open to global conquest. A smart subversion, a great supervillain plan, and a potentially amazing film.
When a corporate criminal pleads insanity in the DC Universe, the joke's on him: it works, and he's sent to Arkham Asylum, where he's gradually driven insane and becomes a supervillain himself. A close look at the inner workings of one of the DCEU's most notorious locales, coupled with an antihero who is, at turns, sympathetic and absolutely despicable. What a fascinating story!