Smallville almost had a Justice League spinoff, which means that long before the days of the Arrowverse, The CW almost had a very different shared universe of characters from DC Comics. The series would have featured a handful of characters from Smallville, but unfortunately it never moved forward.
The Superman prequel series focused on a young Clark Kent's journey as he struggled to accept his destiny, and for the most part leaned on characters from Superman comics, like Lex Luthor, Lois Lane, and Lana Lang. However, over time the series branched out and incorporated heroes and villains from the wider world of DC Comics, beginning with season 4's Bart Allen aka Impulse (Kyle Gallner), who was the fourth incarnation of The Flash in the comics. In season 5, Smallville brought in Aquaman (Alan Ritchson) and Cyborg (Lee Thompson Young). Season 6 introduced Green Arrow (Justin Hartley) as a recurring character.
Smallville season 6, episode 11, titled "Justice", established the Justice League, with Green Arrow at the helm. Oliver Queen put the team together to launch an attack on one of Lex Luthor's facilities. At the end of the episode, the team left town with an open invitation for Clark to join them in the future. This was Oliver's last appearance in season 6. According to longtime Smallville writer Steven DeKnight in an interview with KryptonSite, "Justice" was supposed to set up a Justice League spinoff. The spinoff would have played off the ending of "Justice" by continuing the story of Oliver and his new team. The Smallville season 6 finale would have provided even more setup for the show by having Green Arrow return to help Clark (Tom Welling) in his final battle with Bizarro.
DeKnight has said that Oliver and the others would have lived in Metropolis, with Oliver providing refuge for superheroes "in a Professor X kind of way". This would have included the team used in Smallville and a "couple other DC characters". In "Justice", it's revealed that Oliver saved Impulse, Cyborg, and Aquaman from bad situations, and this is something that he would have done for others in Justice League. DeKnight has chosen not to reveal his entire plan for the show, but he has confirmed that the villain would have been Brainiac, but not the same character portrayed by James Marsters in Smallville.
DeKnight was to become the co-creator and showrunner of Justice League, which he says everyone was excited to make happen. However, despite the enthusiasm that everyone involved had for Justice League, the Smallville spinoff never took shape. This could explain the direction that Smallville chose for Oliver Queen and other members of the team; Green Arrow and the Justice League disappeared after their one-episode team-up, and it wasn't until season 8 of Smallville that the show made Justin Hartley a series regular. At this point, the plans for Justice League had already fallen through - but it's interesting to think about what might have been.