When it came to adapting fan-favorite DC Comics heroes and villains for Young Justice, only four characters were initially off-limits.
In a recent fan Q & A, showrunners Greg Weisman and Brandon Vietti revealed that they were not allowed to use Darkseid, Wonder Girls Donna Troy and Cassie Sandsmark and Blue Beetle Ted Kord while working on Season 1 of the show. "We have no idea why those particular characters were off-limits," Weisman said, "And obviously those restrictions were eventually lifted, as all four have since appeared in the series. Since then, there have been no other characters that were declared off-limits."
While none of these characters appeared in Season 1 of the show, three of the four appeared in Season 2 -- Young Justice: Invasion. Cassie Sandsmark was introduced as a member of the Team, going on missions with heroes like Batgirl and Bumblebee, while Ted Kord was revealed to be the Blue Beetle before Jaime Reyes, though he died in a battle against the Light's villains. Darkseid was the final reveal in the finale of Invasion, but the show was cancelled, forcing fans to wait six years before its resurrection on DC Universe -- though it moved to HBO Max for Season 4, Young Justice: Phantoms.
Donna Troy was introduced in the show's third season, Young Justice: Outsiders, as the United Nations ambassador for Themyscira. While she has been shown periodically throughout the show, she has yet to suit up in her Wonder Girl attire from DC Comics.
Weisman and Vietti may not know why those characters were initially off-limits, but it wouldn't be the first time that DC animation was given restrictions. For example, in Justice League Unlimited, the show was not allowed to use Aquaman villain Black Manta in Season 3, prompting the creation of Devil Ray as a Black Manta substitute. Devil Ray was recently introduced into the main DC Comics continuity in the Black Manta miniseries.
With their restrictions lifted after Season 1 though, the two showrunners have greatly expanded the show's roster of heroes across Seasons 3 and 4. Outsiders not only introduced Donna Troy, but also fan-favorites like Cassandra Cain/Orphan, the Doom Patrol and the Legion of Super-Heroes, with the opportunity open for characters like Jason Todd's Red Hood in the future. In the case of Cassandra, her backstory was explored in the Artemis arc of Phantoms, while some of the Legion has appeared throughout the season.
Phantoms recently returned to HBO Max with the first three episodes of a new arc focusing on Kaldur'ahm/Aquaman. It is unclear if the current season will introduce more heroes and/or villains, but Vietti joked that fans may one day see the Wonder Twins join the series' roster.
The first three seasons and more than half of Young Justice: Phantoms are now available to stream on HBO Max.
Source: DC Comics