The DC Extended Universe is a complicated one, there's no doubt about it. Their Superman is in limbo, their new Batman is possibly in a universe all to himself, and their last movie just bombed at the box office (I'm looking at you, Harley Quinn). That being said, they've had some success too, regarding the likes of Wonder Woman, Shazam and Aquaman. Even Joker, which wasn't in the DCEU, still was a massive hit and a box office explosion for Warner Bros. So what's the next big thing they should focus on?
The ringslingers, otherwise known as the Green Lantern Corps. Now, there is a Green Lantern Corps movie in development being written by Geoff Johns, as well a TV show for HBO Max. No casting rumors or directors yet, but it's the idea that counts. Nevertheless, a universal police force from space is something anyone would being interested in.
For the movie, there were reports that the two main characters for the movie would be Hal Jordan and John Stewart. Hal would provide the veteran mentorship role, while John would be the rookie learning the ropes. So basically, a buddy cop film with superpowered rings. It's definitely the way to go, after the first GL film failed miserably with just Hal as the main focus. Also, the first movie was solely about Hal, while this new one will put more attention to the entire Corps, more or less.
Besides the Green Lanterns simply being interesting characters, there's so much material DC could work with here. You could focus on John Stewart for starters, then make sequels revolving around Jessica Cruz, Kyle Rayner, and Simon Baz. All of which, are ringslingers from the comics. Guy Gardner would give some great comic relief on the side, but even he can develop an arc as time goes on.
However, the GL Corps aren't the only ones out there. Red Lanterns, Yellow Lanterns, and Blue Lanterns are just to name a few that come to mind. There are about 10 different colored Corps from the comics, and they all represent the power of an emotion.
So really, even though a GL Corps movie would be in the DCEU, the material is plentiful enough to hold its own universe. There's just a goldmine of storylines DC could pick from, and if done well, could be a huge hit. With the Green Lanterns already being the embodiment of willpower, making a strong theme and message for this movie would be a cakewalk.
Cast wise, I'd personally go the likes of Michael Fassbender for Hal Jordan, and Daniel Kaluuya for John Stewart. Both of them are decently known names who are great actors, that could use a franchise to call their own.
Overall, DC should really take the Corps and put them up on their biggest stage. I think it's been quite some time since the last GL film left a stench, so it's time to reboot and follow suit.