Crisis On Infinite Earths allows Green Lantern to finally appear in the Arrowverse, thanks to Smallville joining the crossover. The Crisis will span Supergirl, Batwoman, The Flash, Arrow, and DC's Legends of Tomorrow. All of the superheroes, including Smallville's Superman, Tom Welling, will have to unite to save the Multiverse from a cosmic being called the Anti-Monitor (LaMonica Garrett) - and this could bring a certain Emerald Gladiator and possibly even the Green Lantern Corps into the Arrowverse because they're all part of Smallville's universe.
Smallville ended its 10-season run in 2011 with Clark Kent finally donning his Super suit and becoming the Man of Steel. But the adventures of Tom Welling's Superman were chronicled for several more years in DC Comics' Smallville Season 11. Written by Bryan Q. Miller and drawn by a cavalcade of artists, Smallville Season 11 was an official in-canon continuation of the TV series that saw Clark, Lois Lane, Lex Luthor, Chloe Sullivan, Oliver Queen, and Tess Mercer meet Smallville's versions of Batman, Wonder Woman, and the Titans. Uninhibited by the real-world budgetary restrictions of producing a TV show on The CW, Smallville Season 11 let Superman have truly cosmic adventures and nearly unlimited DC Universe guest stars - including Clark not just meeting but briefly becoming Green Lantern!
In the Smallville Season 11 story "Lantern", a Green Lantern Power Ring selected Superman to be worthy to be its wearer. Transformed into the Green Lantern of Sector 2813 (the space sector Krypton belonged to), Superman sought the help of Earth's Green Lantern, John Stewart, and he not only met other ring-slingers like Kilowog and Salaak, but Clark learned about the Guardians of the Universe, the Manhunters, and the Yellow Lantern Corps. Superman also received a download of Smallville's version of Green Lantern history, including the devastating Parallax War that decimated the Corps and caused the deaths of Green Lanterns Hal Jordan and Kyle Rayner. Superman, Stewart, Batman, and the other superheroes then defended Earth from an invasion by Parallax and his Yellow Lanterns before the Man of Steel surrendered his ring and resumed just being Superman.
Meanwhile, Green Lantern is the most significant DC superhero missing from the Arrowverse's Earth-1, where Green Arrow (Stephen Amell) and The Flash (Grant Gustin) live, and Earth-38, which Supergirl (Melissa Benoist) and Superman (Tyler Hoechlin) defend. The possible existence of Green Lantern in the Arrowverse has been long been teased: Coast City (Hal Jordan's hometown) and Ferris Aircraft (owned by Jordan's love interest Carol Ferris) have been referenced but there's been no sign of any Emerald Gladiators. However, during last year's Elseworlds crossover, The Flash of Earth-3 (John Wesley Shipp) asked John Diggle (David Ramsey) where his ring was, implying that an alternate version of Diggle was Earth-3's Green Lantern, possibly John Stewart.
So, Green Lanterns definitely exist elsewhere in the Multiverse and that includes in Smallville's universe. With Tom Welling's Superman and Erica Durance's Lois Lane joining the crossover, Crisis On Infinite Earths promises to update fans on the last 8 years of Smallville's Superman's life. If Smallville Season 11's stories hold as canon in the Arrowverse, it means that Welling's Kent has met three of the biggest Justice Leaguers that haven't been seen in The CW's Multiverse: Batman, Wonder Woman, and Green Lantern. And, while the Crisis may not directly deal with Green Lantern, it's possible that fans might get a glimpse of one or more of the Emerald Gladiators fighting to save Smallville's universe (or other parallel Earths) during the crossover.
Because Smallville had such a long TV run and delved even more deeply into DC lore in its comic book sequel series, in many ways Tom Welling's Superman-centric reality is a more fully-realized DC Universe than the Arrowverse is. And while Welling and Erica Durance are the main Smallville players announced for Crisis On Infinite Earths, the Arrowverse seems to be going for broke to make this event truly epic - and that could include some surprises like a possible sighting of Green Lantern.