Much of the upcoming Green Lantern show on HBO Max is up in the air, with actual concrete information about the plot and all of the characters involved being scarce. The series has only recently begun production, which leaves a lot of time still to add small roles and side characters.
While the talent portraying these roles is still up for debate, one actor who should definitely be involved is Jack Black. This casting might sound completely random, but the actor does have a somewhat tentative history with the Green Lantern franchise. Combine that with his current resurgence in popularity, and even a small cameo for the actor could be a big source of fanservice. Here's history as an Emerald Guardian of Oa and how it could factor into the upcoming HBO Max series.
Jack Black's Green Lantern
Jack Black was initially the choice to play the Green Lantern in a film adaptation before the eventual Ryan Reynolds movie was made, but he wouldn't have been Hal Jordan or any other familiar character from the comics. Instead, he would have been an original character named Jud Plato. Plato was a slacker with a sense of humor similar to Black's own, and the rest of the film would have been much more puerile than fans would have expected. The script was incredibly irreverent to the source material and to the superhero genre in general, and if it was made, it almost certainly would have been an unmitigated disaster beyond even the poorly-received 2011 film.
This dodged bullet still gives Black some apocryphal connection to the IP in a similar way to Nicolas Cage and the unmade Superman Lives. Black is also more popular now than he's been in years, attaining a sort of meme status through the internet like the wave that Keanu Reeves is currently riding. While this isn't enough to make Black the DC Extended Universe's Hal Jordan, it could be used to parley him into another role in the upcoming TV series.
Jack Black in HBO Max's Green Lantern
One common complaint in the current Green Lantern comics is that there are far too many human Green Lanterns, and casting Black as any one of them would probably be a bridge too far for what many would see as a joke. A better idea would be to use Black's voice for a CGI or costumed alien Lantern. One obvious choice in this capacity might be the Green Lantern G'Nort, who's a dog-like alien. His look would be humanoid enough to easily create with mere prosthetics, and his character would actually fit to be portrayed by Black. G'Nort is sort of a lovable loser of the Green Lantern Corps, with many underestimating him for his goofy appearance and personality. If given a prominent role, his journey could be a character arc that pays off when G'Nort steps up and becomes a hero in a big way.
Another possibility might be the even more ironic Jack T. Chance, who's a more humanoid yet still alien Green Lantern. The name would be the biggest link, with the cool, action-hero being a bit outside of Black's usual body of work. His current popularity on the internet could land him the role, however, making it similar to Keanu Reeves' Johnny Silverhand. Or, in an ironic twist of fate, the Jud Plato character could be formally introduced but ultimately passed over as a Lantern, echoing the unmade movie.
Even a mere cameo from Black as himself could work, as the scope of the series will obviously allow for tons of fan service and extra spotlights that the runtime of a movie would not. Whether or not HBO Max's Green Lantern finally grants Black an emerald ring remains to be seen, but missing out on this opportunity would haunt the powers that be forever.