Nearly six years into its existence and the Arrowverse continues to expand. New characters debut every so often and classic locales in DC history crop up on occasion. It’s a treat to watch such moments unfold across Arrow, The Flash, Supergirl, and Legends of Tomorrow. However, what Warner Bros. and DC are willing to give, they will just as quickly take away. This sentiment has been most evident in characters that are apparently reserved for the DC Extended Universe’s exclusive use. In many respects, these unfortunate instances have caused the Arrowverse producers to relinquish plans. The abandoned Suicide Squad plot during Arrow season two offers one such example.

The confusing rules that govern character usage on The CW aren’t the only reasons the four shows may have to avoid using certain heroes and villains. Sometimes the cost to make a character fully functional on-screen tends to get in the way. Similarly, practicality with regards to a character’s live-action appearance is also a variable that may thwart attempts at bring them to the small screen. Either way, examining who can and cannot feature in the Arrowverse is a worthwhile venture we’re ready to embark on. Here are 15 characters The CW’s superhero slate may never use.

15 GREEN LANTERN

Green Lantern

Years ago, hopes were high that the Green Lanterns in some form would make an appearance on either Arrow or The Flash. Many posited that should this occur, John Diggle would be worthy of a Lantern Ring. These hopes then shifted to Legends of Tomorrow, upon the show debuting the Justice Society of America. At the time, Alan Scott/Green Lantern’s appearing on the show seemed a sure thing.

Clearly, such fortune has yet to grace any of the four Arrowverse series.

The hero and all the various elements accompanying his existence are presently tied up in the DCEU. Therefore, it’s unlikely The CW will ever get a chance to explore the Lanterns in any capacity. This is one character fans of the Arrowverse appear to have accepted as firmly off limits.

14 WONDER WOMAN

Gal Gadot's Wonder Woman

Themyscira debuts early in Legends of Tomorrow’s third season; the Amazonian island becomes the home of Helen of Troy. Upon the character’s return in the season three finale, Helen is seen donning garb inspired by the Gal Gadot-starring 2017 film. Despite this, however, no one expects Wonder Woman herself to make an actual appearance on The CW.

Of course, the nebulous rules that govern who can and who cannot exist in both the Arrowverse and the DCEU fuels this assumption. Why can The Flash and Superman be shared between two franchises, while others are either stripped away from DC TV or presumed to not appear? Unfortunately, these questions and the like are sure to never receive an answer. Thus, the few teases of Wonder Woman’s existence in the Arrowverse will have to be pleasing enough.

13 DARKSEID

Darkseid in Darkseid War

Like others, Darkseid is bound to be a character the DCEU will not permit to appear on television and film concurrently. He further complicates an already complicated situation, given his connection to the New Gods. A New Gods film helmed by Ava DuVernay, while exciting, remains shrouded in mystery. With conflicting reports as to whether the film is a DCEU entry, we’re left to wonder if this means two different versions of the Apokoliptan ruler will appear on the big screen in the near future.

Regardless, this spells doom for any chance of the Arrowverse introducing the character.

As such, the rest of the New Gods’ compelling lore is probably off the table as well. It’s unfortunate; Darkseid, and any number of the characters that surround him, would make quite the formidable foe, specifically on Supergirl.

12 DEADSHOT

Deadshot on Arrow

As the release of Suicide Squad drew near in 2016, Arrow had to rid itself of characters associated with the Task Force. The assassin Floyd Lawton/Deadshot is among the loses, a character who is still deeply missed in the series. While his arc on the show is one of the most revered, fans wanted more, expected more.

Fortunately for the character, he goes out like a hero by sacrificing himself to save others. It’s an honorable act, one even John Diggle acknowledges and admires. Thus, Deadshot receives closure on the series, which is generally well-liked among audiences. But knowing that other plans may have elevated the character further is endlessly disappointing. Perhaps he’ll appear on another Earth in the not too distant future, as he did during The Flash’s sophomore season.

11 SWAMP THING

Swamp Thing

Swamp Thing has been a fan favorite hopeful addition to the Arrowverse for years. Now that Matt Ryan’s John Constantine will appear in Legends’ fourth season as a series regular, these hopes are at an all time high. The question then arises of whether Swamp Thing could work on a television budget.

It seems reasonable, especially when considering Gorilla Grodd, King Shark, and the recent reveal of Mallus’ demon form.

However, Swamp Thing is different from all of the above. King Shark’s speech is limited. Grodd speaks through telepathy. Mallus is a demon; who cares how he talks? Swamp Thing, though, speaks regularly and does so in a manner befitting humans. Can the technology that powers Grodd present a massive form of greenery in a defined shape that can speak? And make it look good? There’s no doubting the Arrowverse’s tech department, but Swamp Thing may be one dream that's out of reach.

10 LEX LUTHOR

Jesse Eisenberg as Lex Luthor

To be fair, Lex Luthor made his official Arrowverse debut during Supergirl season two. He is seen as a child, though, in a flashback about Lena Luthor’s formally being taken in by the family. As an adult, the Superman rogue has yet to appear, which is somewhat surprising when considering how often characters in the series mention him.

Evidently, what keeps him away are his ties to the DC Extended Universe. But, again, the seemingly arbitrary rules make understanding his case difficult. If Superman’s fair game for both film and television, why not his villains, too? Perhaps Lex will one day feature on an episode of Supergirl; it's not out of the realm of possibility. Yet, as things presently stand, it looks as though this will not happen anytime soon.

9 ONOMATOPOEIA

Onomatopoeia

This Kevin Smith-created character, who speaks in onomatopoeias, has been requested to appear on Arrow since Smith himself suggested it could work several years ago. Can it work, though? Yes. Can it be done without coming across as campy, a tone Arrow appears to desire steering clear of? Debatable. To speak in onomatopoeias one must verbalize a sound with words; i.e., saying “splat,” “crack,” or “boing” in lieu of making the actual sound.

Again, this is practical in live-action -- an actor would simply be required to speak in this unique fashion.

However, in addition to concerns of Onomatopoeia being construed as silly, there is the problem of whether he can be conveyed on-screen as elegantly as on the page of a comic. Maybe one day audiences will know for sure.

8 BATMAN

Ben Affleck's Batsuit

Arrow and Supergirl have more than had their fair share of Batman teases. On both Earth-1 and Earth-38 Bruce Wayne is most definitely active as a vigilante. Yet, of course, the character has yet to physically feature on either of the shows. That this will ever change seems so unlikely that even producers in the Arrowverse have attempted to steer fans away from pondering the possibilities.

Interestingly, though it is certainly a factor, Batman’s status as DC’s and WB’s golden boy may not be the only thing keeping him from a CW debut. The character’s television rights are properties of Fox. Therefore, the Bat suiting up in live-action on the small screen, at least in the near future, is presumably exclusive to Gotham. Additionally, Disney’s forthcoming purchase of Fox may alter rules and rights in a manner that remains unclear to the public.

7 DEATHSTROKE

Deathstroke on Arrow

Deathstroke’s on-again, off-again relationship with the Arrowverse has been as tumultuous as the character’s rocky friendship with Oliver Queen. Slade Wilson’s been an Arrow staple since season one. After season two, his usage diminishes for reasons that were then unknown. Eventually, the character’s introduction into the DCEU made things a little clearer. Fortunately, his return during Arrow’s fifth and sixth season offers his arc much needed closure.

Unfortunately, it's met with news of his being relegated to the sidelines once more for purposes of the Terminator’s cinematic ventures.

The permissions concerning Deathstroke’s featuring in two separate universes sparks more confusion than any other DC character. Apparently, there are some details the public isn’t meant to fully understand. Regardless, here's to hoping Arrow hasn’t seen the last of Slade.

6 BOOSTER GOLD

Booster Gold

A live-action film starring Booster Gold has been in the works for years. Reportedly, this project is still moving along, albeit rather slowly. Could this thwart the hero from potentially making an Arrowverse appearance in the future? It is possible. However, even before news of a Booster Gold film surfaced, his addition to this iteration of DC television seemed little more than a dream.

Every season of Legends of Tomorrow that passes without Booster getting so much as a cursory mention solidifies the unlikelihood of his debuting. Furthermore, apart from the existence of Rip Hunter, there exists no evidence of Booster Gold even being prevalent in the Arrowverse. This could mean any number of things. Hopefully, it all boils down to his eventually being welcomed aboard the Waverider by the Legends.

5 AMANDA WALLER

Amanda Waller and Arrow's Suicide Squad

The strange circumstances surrounding Amanda Waller’s unexpected death in Arrow season four remains contentious. Undeniably, the character is deserving of a better send-off. The reasoning behind her having to depart the series at all still manages to perplex fans, especially since a few other characters connected to Task Force X were killed off in seasons prior.

Without question, Suicide Squad’s summer 2016 premiere constitutes the basis for Waller’s being written out.

Unfortunately, this means she’s done for good. No alternate Earth versions of her should be expected. She’s a fascinating villain, a strong foil for Oliver and Diggle’s preferred brand of heroism. This makes the plot lines in which she’s a central figure all the more compelling. It’s a shame that such an incredible asset to Arrow is excised for extraneous purposes beyond the show’s control.

4 NIGHTWING

Nightwing

Dick Grayson never receives a direct, or even an indirect, reference on either of The CW’s DC series. However, Blüdhaven, the city in which Grayson protects as Nightwing, has earned a mention or two on Arrow. Evidence elsewhere suggests this version of the character does exist in the Arrowverse, though (i.e., the constantly teased existences of Batman and Oracle). Should fans then expect the former Robin to appear? Absolutely not.

Again, the mystifying rules that govern character usage will maintain Nightwing’s being elusive for the Arrowverse. This especially rings true because he’s integral to other live-action universes -- Titans and the Nightwing solo film that’s still in early development. His ever appearing on one of the four shows is one dream that was long ago left to the wayside.

3 CAPTAIN BOOMERANG

Nick E. Tarabay as Captain Boomerang on Arrow

Despite making several Arrowverse appearances, this classic Flash rogue has never featured on an episode of The Flash. All of actor Nick E. Tarabay’s ventures place him on Arrow, nothing else. On one hand, The CW deserves applause for developing a series of shows where these puzzling circumstances can occur. On the other, it’s frustrating. Does Captain Boomerang fail to star in a Flash episode because of the DCEU? Or is he considered a better fit for Arrow?

Certainly, his limited use and supposed death gives credence to the film universe having the character on reserve.

But an argument could be made that this version of Digger Harkness makes for a more interesting Arrow foe/ally. His presumed death on Lian Yu during the season five finale may eliminate the possibility of his return. With Suicide Squad 2 in the works, dead or not, he’s unlikely to reappear anytime soon.

2 AQUAMAN

Jason Momoa's Aquaman

In The Flash’s second season, Aquaman receives his first and only tease in the Arrowverse. During the episode “Escape from Earth-2,” that Earth’s Iris West mentions having relatives in Atlantis. Presumably, Arthur Curry/Aquaman is King of the alternate universe’s underwater metropolis. Since then, neither the DC locale nor Aquaman has received another tease. Like other characters on this list, the live-action film universe thwarts Aquaman’s potentially featuring in the shows.

In truth, however, it is difficult to see how the Atlantean hero would fit even if he were allowed to make his Arrowverse debut. On which show would he guest star and what kind of story would befit his characterization? That the answers to these questions will never earn exploration is unfortunate to say the least.

1 HARLEY QUINN

Margot Robbie as Harley Quinn

After she makes her Arrow debut during the show’s sophomore effort, fans were eager to see what would become of this version of Harley Quinn. The potential could not be made any more alluring. Is a place on Amanda Waller’s Task Force X in her future? How do her and Joker separate? These questions and countless others continue to brew in some shape or form.

Apparently, Arrow producers were ready and willing to answer them.

In a disappointing turn of events, all of the show’s ties to Harley and the rest of the Suicide Squad are axed not too long after they begin developing in earnest. This was the first example of the film series encroaching on the television universe’s territory. As evidenced by Deathstroke’s recent and indefinite removal from the series, it is not and will not be the last time.