Can Smallville Be Retconned As Supergirl's Past?
Another possibility for Smallville's Arrowverse prospects is to retcon the series as Tyler Hoechlin's Superman's canonical past on Supergirl... but this scenario is vastly more complicated and ultimately wouldn't work with Supergirl's continuity. There is just far too much conflict between Supergirl's canon and Smallville, not the least of which are all of the Smallville actors who have appeared on Supergirl. Along with Erica Durance, who recurs as Kara's Kryptonian mother Alura, Laura Vandervoort, who played Smallville's Supergirl, appeared as the villainous Indigo in season 1. Also, Sam Witwer, who played Doomsday in Smallville season 8, is now National City's newest villain, Agent Liberty.
Meanwhile, the sheer amount of Supergirl canon at odds with Smallville is staggering, and that starts with James Olsen (Mehcad Brooks), who is African-American (as opposed to Smallville's Jimmy Olsen played by Aaron Ashmore). Supergirl's Cat Grant (Calista Flockhart) is more influential and powerful than Smallville's gossip columnist (Keri Lynn Pratt) ever was, and Supergirl's Legion of Superheroes is very different from Smallville's, with Alexz Johnson's Imra on Smallville totally conflicting with Amy Jackson's Saturn Girl on Supergirl. Also, General Zod (Callum Blue) was Smallville season 9's Big Bad, and he's totally different from Supergirl's Zod (Mark Gibbon). Then there's the entire Luthor family, who have all appeared on Supergirl (with Lex on the way) and are established to be different from Smallville's nefarious clan. In fact, Lena Luthor's (Katie McGrath) life story diverges quite a bit from Cassidy Freeman's Tess Mercer, whose real name was revealed as Lutessa Lena Luthor.
Supergirl has definitely given shout-outs to Smallville, however, and has woven aspects of that show into their Superman's past. The most notable mention was "Clark's friend Chloe" in season 3 - a sly reference to Chloe Sullivan (Allison Mack) and her Wall of Weird. Perhaps the biggest reference is still to come in Elseworlds, since set photos reveal Kara visits Clark and Lois Lane (Elizabeth Tullock) at the Kent Farm in Smallville, Kansas. And while the possibility exists for other major aspects of Smallville to become part of Supergirl's lore - it could be fun, for instance, for Justin Hartley's Green Arrow to be Earth-38's Emerald Archer as opposed to Stephen Amell - a straight-up retcon that merges the two canons together just wouldn't fly.
Why Smallville Deserves To Be Part of the Arrowverse
Smallville is one of the most groundbreaking and influential live-action superhero shows ever, especially to the Arrowverse series that followed it. Before Smallville, DC's superhero shows were mostly campy affairs like Adam West's Batman or Lynda Carter's Wonder Woman, while the Tim Burton-inspired The Flash 1990 only lasted one season. By contrast, Smallville endured for a decade as the WB/CW's signature show. By making the characters young, hip, and sexy (a strategy the Arrowverse has echoed), the series redefined Superman for a generation, all without Tom Welling actually donning the Man of Steel's colorful costume.
Also, it's Smallville that pioneered the kind of fan service the Arrowverse regularly exemplifies. In season 2, Tom Welling's series openly acknowledged their predecessors when they cast Christopher Reeve as Dr. Virgil Swann and brought with him the iconic John Williams score from Superman: The Movie. Other actors from film and TV followed and soon, most of the Superman films' iconography became official parts of Smallville canon, including the ice crystal Fortress of Solitude, the Phantom Zone, and even Margot Kidder guest starring as Dr. Swann's assistant Bridget Crosby. When Welling's Clark Kent finally received his Super suit, it was the same uniform worn by Superman Returns' Brandon Routh (who's now the Atom on DC's Legends of Tomorrow).
When it comes to honoring the superheroes that blazed the trail, Smallville did it all first and its success made the Arrowverse possible. Now the Arrowverse has the means to follow suit and make the iconic series a part of their Multiverse. Although there could be legal issues regarding Smallville's rights that may have to be overcome (it's notable that Smallville isn't currently offered on the DC Universe streaming service), the Arrowverse's producers have proven adept at working such miracles - see John Constantine (Matt Ryan) moving beyond his failed NBC series and joining the Arrowverse. Establishing that Smallville is one of the Arrowverse's 53 Earths would only enrich the existing CW series while offering new and exciting creative possibilities. Most importantly, it's something fans definitely want.