It may have been a long time coming and has received a downgrade from theatrical movie to HBO Max series, but it looks like the for fans to forgive or forget 2011's Green Lantern movie.
With over sixty years of storylines to draw from Green Lantern Corps has plenty of options, especially when it comes to villains the Corps can face. They'll need to be powerful enough to challenge an army of superhero cops wielding pure imagination, so they'll all be pretty intimidating.
Sinestro Corps
By far the most significant villain that Hal Jordan's Green Lantern has ever faced is Sinestro, the fallen Lantern who wields the yellow light of Fear. Eventually, his own Sinestro Corps would recruit enough followers to declare all-out war against the Green Lantern Corps, a war that raged between Oa and Earth and one that the Lanterns (fighting alongside Earth's heroes) only narrowly won at the time As the Green Lantern post-credits scene revealed, the movie's sequel was supposed to feature Sinestro (Mark Strong) turning evil, so it would be perfect for the new series to kick off with this plotline.
Darkseid
There is one big question hanging over Green Lantern Corps, and that's whether or not it's set in the DCEU. If it is, then it's uniquely positioned to carry on Zack Snyder's Justice League and bring the DC Universe's ultimate cosmic evil, Darkseid, in as a villain. The movie concluded with Darkseid readying his armada to invade Earth, so if Warner Bros isn't keen on doing Justice League 2 it would be fantastic if Darkseid was intercepted and countered by the Green Lantern Corps instead - especially as a Green Lantern was present during Darkseid's initial defeat on Earth.
Atrocitus And The Red Lanterns
After the Sinestro Corps, the Red Lanterns of Rage represent the biggest direct threat to the Green Lantern Corps. Their leader is the monstrous Atrocitus, the sole survivor of Sector 666, which was completely wiped out by the forerunners to the Green Lantern Corps - the robotic Manhunters.
As such, Atrocitus has a deep grudge against the Guardians of Oa and constantly seeks their extermination. The Red Lanterns were the main villains of Green Lantern: The Animated Series, which launched as a semi-spin-off of the movie, and it would be great to see them in live-action too.
Larfleeze
Larfleeze is one of the most distinct enemies that the Green Lantern Corps has ever faced. Unlike every other army of the emotional spectrum, the Orange Lanterns of Avarice have only one member: Larfleeze. Larfleeze, otherwise known as Agent Orange, has a limitless capacity for greed and nothing much else. He needs to possess everything, including living beings, and anyone he kills becomes one of his constructs. Considering he's just one person and is often played for comic relief, Larfleeze is exceptionally powerful and has killed many Green Lanterns over the years.
Parallax
Anyone who's seen the 2011 Green Lantern movie may recognize the name Parallax as the much-reviled main villain. The movie's Parallax didn't work because it was so vastly different from the more disturbing comics incarnation. It's known in two major forms. The first was a corrupted Hal Jordan, which would be an interesting villain for Green Lantern Corps if he's not the main character. Parallax's second, true form is a parasitic embodiment of fear, a nightmarish yellow creature that can completely overcome any host who wields its power.
The Darkstars
The Guardians of Oa were originally part of an ancient race called the Maltusians, but they were not the only ones from Maltus who created an army to bring order to the universe. An isolationist sect called the Controllers created the Darkstars (or Blackstars) to patrol the galaxy and bring it to order. When the Green Lantern Corps collapsed (thanks to Hal Jordan as Parallax), the Darkstars tried to replace them but failed. In more recent times, the Controllers began kidnapping Guardians to turn them into themselves but were undone when their new improved Darkstars turned on them. These new Darkstars would pose a formidable threat to the Lanterns.
The Star Sapphires
Originally the Star Sapphires were representatives of the immortal Zamarons, a race of warrior women that are basically an alien version of the Amazons of Themyscira. The most famous Star Sapphire is Carol Ferris, the occasional love interest of Hal Jordan, who becomes a villain after he lets her down one too many times.
More recently the Star Sapphires became a full Lantern Corps to match the Green Lanterns, with the violet power of Love. They're not always villainous, but they're not always to be trusted either.
Brainiac
If there's one major villain that DC fans have been waiting years to see on the big screen, it's the Coluan world-destroyer Brainiac. Brainiac has been planned for multiple canceled Superman movies, including movie instead, however.
The Manhunters
Before the Guardians of Oa harnessed the power of Will to create the Green Lantern Corps, they believed an emotionless army of robots was the best way to maintain order across the universe. Eventually, the Manhunters rebelled against the Guardians and destroyed Sector 666 and all of its inhabitants. While the Manhunters were originally thought destroyed following this incident, they survived and have returned to threaten the Green Lanterns on multiple occasions - including Green Lantern: The Animated Series and the videogame Rise of the Manhunters, both of which served as sequels to the movie.
The Black Lanterns
One of the biggest threats the DC universe has ever faced was the Blackest Night, where an army of the dead rose across the galaxy to destroy all life. This army was controlled by the human villain Black Hand and the Black Lantern Ring granted to him by Nekron, the lord of the dead. The Blackest Night comics event saw every hero and villain who had died return, more powerful than before. Eventually, only the combined powers of all the various Lantern Corps were able to defeat them. That then led to White Lanterns and the Brightest Day event, which caused further problems for the Green Lantern Corps.