Considering the emergence of Rocket, Icon, and Static in Young Justice, it wouldn’t be shocking that the premier villainous faction of Dakota City would make the transition. Formed from surviving members of various gangs following the Big Bang (an event which unlocked the metagene of several people in Dakota), the Blood Syndicate started out as a bloody anti-hero team until coming under the leadership of Holocaust, arguably the biggest bad in the city. The group would run afoul of Icon, Static, Hardware, Harm, and the various other heroes in Dakota (including several of the Syndicate’s founding members) as their second leader Wise Son would return to battle Holocaust for the team’s control. For YJ, its been alluded that many of the villains from the second season have been rounded up and their prison is no longer under the control of the Light (meaning, they couldn’t come and go as they pleased anymore). As such, the series would likely require new villains to emerge for the heroes to face as pawns of the Light making the Blood Syndicate optimal. It’s also possible at this point even WildStorm characters maybe available to the series so other groups such as the WildC.A.T.s’ Cabal or Gen¹³’s Dv8 could emerge.
Formed by Deathstroke as a villainous mirror of the Teen Titans, Titans East represented some of the heroic group’s greatest young foes like Match, Inertia, and Cassandra Cain (whom was manipulated by Deathstroke). The group itself was short lived, resurrected in a manner in groups like Clock King’s Terror Titans, Superboy-Prime’s Legion of Doom, and Deathstroke’s Titans (though the latter may have had more in common with Vandal Savage’s earlier group Tartarus made up mostly of adults, save Red Panzer). In the second season of Young Justice in Invasion, the Light remark on producing its own version of the Team using the test experiments of their then partner the Reach. The group would be clued in by new friend Arsenal that they were being used, thus dropping this avenue for the Light. Such then, it’s possible the villainous group may look within its ranks for its own Team. Children and relatives of villains are not uncommon, such include Lex Luthor’s niece Nasthalthia and daughter Lena, General Zod and Ursa’s son Lor-Zod, Brainiac’s creation Indigo, Vandal Savage’s daughter Scandal, Deathstroke’s children Jericho and Ravager, Two-Face’s daughter Duela (aka Joker’s Daughter and Harlequin), Riddler’s daughter Enigma, Sinestro’s daughter Soranik Natu, Black Adam’s brother-in-law Osiris, Queen Clea’s daughter Ptra, Madame Rouge’s daughter Gemini, Bolt’s son Dreadbolt, and Captain Boomerang’s son Owen Mercer (at one point, the anti-hero Anarky was considered to be the Joker’s son). So, there’s certainly ground to be mined. Of course, YJ has already introduced Icicle, Jr. into the series which provides a mentor figure for such a unique group. But, what’s more interesting, many of these characters went on to become heroes or anti-heroes which means their story could provide an interesting character journey within the series (especially as you consider in the comics, it’s Tigress who marries Icicle, Jr).
For some time, Young Justice has teased Mars. Its culture has been discussed, glimpses of it have been seen, beings from its world have emerged on Earth, and yet we’ve not traveled there thus far. In the final episode of the series, in its final moments, it’s mentioned the former Green Beetle B’arzz O’oomm has requested the Team’s help on his homeworld. Noted earlier, those behind the scenes of the show rarely provide something in the series without reason. It’s then possible the third season could finally approach Mars. On the show, it’s noted racism persists as White Martians are viewed as the inferior color of the species despite being a race of shapeshifters. In the comics, this was also so but for different reasons. In the comics, the Whites were seen as warmongers, primal examples of their species in the face of the philosophical, peaceful Greens (where Yellows were rarely discussed beyond the Silver Age and White and Red Saturnians were an offshoot of Mars who cohabited with the Faceless Hunters). Miss Martian, a White-skinned member of her race, left her planet to escape civil war on Mars. Later learning of Martian Manhunter on Earth, she came to the planet and masqueraded as a Green Martian and superhero. White Martians are a common foe of Martian Manhunter in the comics, where White Martian General Blanx destroyed Mars before the Crisis (after Crisis, Manhunter’s twin brother Ma’alefa’ak was responsible) and a band of surviving White Martians called the Hyperclan would try to conquer Earth after Crisis. Led by Protex, the group was the advanced guard of a fleet of their kind and nearly succeeded until Batman and Superman realized who they were (and of their weakness to fire) leading to their defeat. They would return from time-to-time until being killed by the Manhunter’s murderous alter ego Fernus. Ma’alefa’ak would later be portrayed as a White Martian in Batman: The Brave and the Bold.
Following a team-up by the Teen Titans and Legion of Super-Heroes, the former end up ten years in the future of their present. In that time period, they discover future versions of themselves who had become villainous versions of their former selves (including an adult Superboy), finding mentors in the likes of Ares and Lex Luthor rather than the Justice League. The adult Titans try to wipe the memories of their younger selves to ensure their future but fail as the youths return to their time via the Cosmic Treadmill. The future group would later come to the present with an army of allies seeking to re-instate their timeline. For Young Justice, there was a point in time where several members appeared they could have turned to the dark side. Miss Martian was blackmailed by Queen Bee to protect her White Martian identity and safeguard the Logan family, Superboy was addicted to “shields” provided by Lex Luthor, and Artemis was blackmailed by her father to keep her friends from learning her villainous familial ties (while Red Arrow was an unwitting spy for the Light). Further, in the second season, Aqualad masqueraded as an enemy (a subordinate of his father Black Manta) and he was later joined in this ruse by Artemis who adopted the guise of Tigress (While Miss Martian practiced the despicable tactic of leaving her foes catatonic following data gathering). It wouldn’t then be challenging to see how even for these five they could have been turned to the Light’s forces. Only turning Robin and Kid Flash (if resurrected) would fully provide for a villainous version of the Team’s founders that could be discovered in the near future.
The Light has employed several aliens in their plans but the ones they could likely benefit the most from joining forces with would be Kryptonians. The Phantom Zone is a still place between dimensions that allows those within to observe the material world but be unable to interact with it (like phantoms). In this place, they do not age, sleep, or hunger, simply observe. Krypton employed the Zone as a prison for its worst offenders, a place to spend several lifetimes to serve for their most grievous crimes before being released generations later. For this, they employed Projectors to open gateways to and from the prison as well as Viewers to observe those within the Zone. Such notable prisoners include General Dru-Zod, Faora Hu-Ul/Ursa, Jax-Ur/Non, Kru-El, and Va-Kox. Over the years, several prisoners would escape the Zone from time-to-time and combat Superman or Supergirl, occasionally several as a group (including one such time they tossed the Justice League satellite headquarters out of orbit with several Leaguers aboard). Post-Infinite Crisis, one storyline would see Zod and his mate Ursa have a child thanks to the special properties of an installation in the Zone called Fort Rozz. Using the technology therein, they manage to create a small hole into the material world and have their child Lor-Zod squeeze through in hopes he would free them. Instead, the child would take the name Chris and was adopted by Clark Kent and wife Lois Lane. Zod and Ursa managed to escape anyway and freed all of the criminals. Superman and the Superman Revenge Squad managed to return the prisoners back to the Phantom Zone but they would be freed again later due to the founding of New Krypton. Escapees of the Phantom Zone (and specifically Fort Rozz) act largely as the main source of difficulty for characters in the TV series Supergirl.
In Young Justice, the position of L-5 within the Light is a veritable musical chair. In the beginning of the series, it was held by Ocean-Master only to lose it later to Black Manta. Near the conclusion of Invasion, Manta is taken into custody. It seems likely then that another Aquaman villain assume the seat with the most likely being one of either Fisherman, Scavenger, or Human Flying Fish. However, where Ocean-Master and Black Manta were something of iconic super-villains, those others mentioned fall a bit short. Instead, the seat maybe best served by a more recent addition to Aquaman’s foes. While Charybdis or Kordax would be viable, Siren could be an interesting addition. Twin sister of Mera, Aquaman’s wife and the queen of Atlantis, Hila was exiled from her home with her lover Kandor. The duo would attack Aquaman years later to locate the mystical Golden Eels but Hila, who was being mind-controlled by Kandor, turned on her lover to help her brother-in-law. Following Final Crisis, Hila would return where it’s revealed Mera was suppose to assassinate Aquaman only to instead fall in love with him. Considering her failure, her sister Hila (now called Siren) was sent to finish the job. Siren’s emergence in YJ could be interesting considering she’s bears such a strong resemblance to Mera (maybe even using the same voice actress) which could be something played up and it would mean another woman on the Light’s council (alongside Queen Bee and perhaps Talia if she steps in for her father). It would also mean an extremely powerful fighter as she bears Mera’s abilities as well which put her over the other candidates mentioned save perhaps Kordax.
Mentioned several times, the minds behind Young Justice rarely do anything unless it means something. So to have had Jason Todd as Robin in the Team during the five years between the first two seasons, it seems unlikely nothing would be done with it. In the comics, Jason Todd was murdered by the Joker and was considered on the short list of characters that would never be resurrected (alongside the likes of Bucky Barnes, Ben Parker, and Gwen Stacy). However, the character would eventually return as part of the event Infinite Crisis as Red Hood. A seemingly new player in the Gotham crime game, Red Hood was an anti-hero who tried to control crime (viewing it as an inevitability that could be operated as a business). Going head-to-head with Gotham’s kingpin at the time in Black Mask, it would be revealed the character was a returned Jason Todd thanks to the reality-altering aspects of Infinite Crisis. Feeling he was unavenged for his death as Batman chose not to kill the Joker, Todd kidnapped and tortured his murderer and lured Batman to where the villain was held. Todd felt if Batman killed the Joker then-and-there, it could make right that wrong only for Batman to choose life as the Joker tried to kill them all together. Since then, a number of stories in other outlets of media emerged featuring the Red Hood. One such project was an animated film adapting his return called Batman: Under the Red Hood (2010) which featured an alliance between Joker and Ra’s al Ghul resulting in Todd’s death leading the latter to use his Lazarus Pit to resurrect the youth in recompense. Another is Batman: Arkham Knight (2015) where the Joker tortured and brainwashed Todd into believing Batman abandoned him and was his true enemy. The Joker convinced Batman he had murdered Todd as the youth was released to spend years plotting revenge on his former mentor in which he returned as the Arkham Knight and aligned with the Scarecrow.