Which superheroes from Smallville haven't been used by the Arrowverse yet? "Crisis on Infinite Earths" technically made Smallville part of the Arrowverse canon, so in a way those characters were integrated into the Arrowverse, but the CW shows could eventually use their own versions.
The two universes share a lot of the same characters, including Superman, Lois Lane, Supergirl, and Lex Luthor. This makes sense, considering that the Arrowverse has its own show set in Superman's world, Supergirl, but Smallville also had a lot of characters that appeared in shows like The Flash, Arrow, and Legends of Tomorrow. Long before any of these shows were created, Smallville had its own Green Arrow, Hawkman, Black Canary, and Martian Manhunter, but there's quite a few that have never been in any of the five Arrowverse shows.
For some of those, Smallville serves as the only movie or show where they've appeared in live-action. Many are characters who could be introduced in the Arrowverse at some point or another. Here are all the superheroes who have appeared in Smallville but never in the Arrowverse.
Bart Allen
Smallville introduced its first major DC Comics superhero outside of Superman comics when it brought in Bart Allen (Kyle Gallner), the fourth version of the Flash from the comics. His guest appearance in season 4 led to Smallville starting a tradition of bringing in at least one new hero every season. The Flash has incorporated multiple versions of the Scarlet Speedster, but there's still no sign of Bart Allen.
Aquaman
Smallville season 5 introduced Aquaman (Alan Ritchson), complete with his orange and green color scheme, as a hero who was interfering with one of Lex's operations because it was a threat to marine life. In a season 10 episode, Aquaman brought along his wife, Mera (Elena Satine), who is his primary love interest in the comics. Aquaman's home, Atlantis, exists in the Arrowverse, but given the importance of Aquaman to the DCEU, there isn't a strong chance that he'll showing up anytime soon.
Cyborg
Not long after meeting Aquaman, Clark (Tom Welling) and Lana (Kristin Kreuk) had a run-in with Cyborg (Lee Thompson Young), a football star transformed into a half-machine, half-human weapon by Lex Luthor's scientists. Along with Bart Allen, Green Arrow (Justin Hartley), and Aquaman, Cyborg helped form Smallville's Justice League in season 6.
Lightning Lad & Cosmic Boy
In Smallville season 8, the time-travelling Legion of Super Heroes visited Clark from the future. Smallville's version of the team consisted of Saturn Girl (Alexz Johnson), Lightning Lad (Calum Worthy), and Cosmic Boy (Ryan Kennedy). Supergirl gave the Arrowverse its own take on the Legion, but of these three, the only character it used was Saturn Girl. Lightning Lad and Cosmic Boy never appeared.
Zatanna
Zatanna (Serinda Swan) was a magician whose powerful magic caused a lot of headaches for Clark in Smallville. Her spell that granted everyone's greatest wish ended in disaster, but in the end Clark found her to be a valuable ally of the Justice League. Zatanna remains one of the biggest characters the Arrowverse has yet to find room for.
Wonder Twins
Clark was faced with a problem when two of his biggest fans, Zan (David Gallagher) and Jayna (Allison Scagliotti), used their super powers to fight crime in Metropolis and ended up doing more harm than good. The trouble they caused trying to imitate Clark's superhero alter ego, The Blur, forced Clark to clean up their mess. Unlike Smallville's other superheroes, Zan and Jayna (better known as the Wonder Twins) didn't originate from the comics. They're actually from the animated series, Super Friends. In "Crisis on Infinite Earths", the Arrowverse included a fun Wonder Twins tease by revealing the cage of their pet monkey, Gleek.
Speedy (Mia Dearden)
Oliver Queen gained his comic book sidekick in season 9 when he met Mia Dearden (Elise Gatien). In the comics, Mia Dearden fights alongside the Green Arrow as Speedy. In Smallville, she was a young woman participating in an underground fight club when she was discovered by Oliver, who took her under his wing to be trained. The Arrowverse doesn't have Mia Dearden, but Oliver's sister, Thea Queen (Willa Holland), is loosely based on her.
Doctor Fate (& Other Members of the Justice Society)
In the comics, the Justice Society is a Golden Age team of superheroes from the 1940s and the precursor to the Justice League. Smallville introduced them in the two-hour season 9 special, "Absolute Justice", with Stargirl, Hawkman, and Doctor Fate representing the team. Smallville used cameos and a large painting to reveal past members of the Justice Society, but most of them are now dead. Legends of Tomorrow has also used the Justice Society, but with a very different lineup. The characters in Smallville's Justice Society that aren't in the Arrowverse are Doctor Fate, the Star-Spangled Kid, Sandman, and the original versions of Atom and Green Lantern. With DC Universe's Stargirl leaning heavily on the Justice Society, fans shouldn't look for these characters to join the Arrowverse in the near future.
Blue Beetle
Smallville season 10 used two different versions of the Blue Beetle from DC Comics - Ted Kord (Sebastian Spence) and Jaime Reyes (Jaren Brandt Bartlett) - but only gave Reyes the alien scarab artifact that turns the user into the Blue Beetle. Clark and Lois tried to get billionaire CEO Ted Kord to remove it, but eventually Reyes learned to control it and the powers that came with it. Blue Beetle is a character who fans have long wished to the see in the Arrowverse, but it still hasn't happened. However, Ted Kord is confirmed to exist in the Arrowverse, as both he and his company have been mentioned several times in episodes of The Flash and Arrow.
Superboy
Superboy aka Conner Kent (Lucas Grabeel) is a Kryptonian-human hybrid clone created from the DNA of Clark Kent and Lex Luthor. After going through a period of accelerated aging, Conner was groomed by a parallel universe Lionel Luthor (John Glover) to be the new Lex, but as the Kryptonian side of his persona started to emerge, Clark and Tess (Cassidy Freeman) discovered that they could save him. Clark brought Conner into his family and even provided him with his comic book accurate black shirt and Superman emblem.
Booster Gold
Booster Gold (Eric Martsolf) made his first and only live-action appearance in the same Smallville episode that introduced Blue Beetle. Booster traveled from the future in the hopes of replacing Superman and stealing his destiny, but a pep talk from Clark and an encounter with Blue Beetle made Booster realize that he was trying to be a hero for the glory, which was the wrong reason. At Clark's suggestion, Booster decided to train Blue Beetle.