Warning: SPOILERS for Superman and Lois season 1, episode 2, "Heritage".
Superman & Lois episode 2 revealed more of Captain Luthor's history and motivations - and potentially set up an Earth-Prime return of the villain Reactron. After recurring throughout the Arrowverse, the Man of Steel and DC Comics' most famous reporter were given the spotlight. The series premiered in February and has already been renewed for season 2. Played by Tyler Hoechlin and Elizabeth Tulloch, respectively, Clark Kent and Lois Lane found their lives turned upside down in the first episode. After losing his job at the Daily Planet and his mother in quick succession, the duo relocated to Smallville. Simultaneously, Superman found himself drawn into conflict with a mysterious stranger, Lois began investigating Morgan Edge, and it emerged that one of their twin sons was superpowered.
Following the reveal that The Stranger was an alternate version of Lex Luthor, Superman & Lois season 1, episode 2, "Heritage" detailed more of his backstory. After Captain Luthor reached out to General Sam Lane, viewers were offered a flashback. As it turned out, Luthor and Lane were friends and allies on the former's original Earth. Together they were revealed to be part of a unit at war with an evil version of Superman. Though Lane was killed, Luthor somehow survived the battle (as well as the multiverse collapse) and made it to Earth-Prime. Trying to connect with this world's Sam Lane, Luthor gave him a set of dog tags. Depending on which way they were held, they either spelled out "7734" or "HELL" — both of which connect to Anti-Kryptonian groups and could further foreshadow a future villain.
In the comics, Project 7734 was the brainchild of Sam Lane. Designed to safeguard the Earth against extraterrestrial threats, 7734 frequently targeted Superman and other Kryptonians. Over time, the group merged with the Human Defense Corps, more specifically Squad K. In pursuit of Kryptonian extermination, Lane actually did align with Lex Luthor. Furthermore, he utilized many monstrous and superpowered agents. These included Doomsday, a murderous Superwoman, and Metallo. Given that Superman & Lois' Captain Luthor had 7734 tags and used the Corps mantra ("To Hell and Back") it would appear that the group had already been merged on his original Earth. And though the Arrowverse's Lane seemed committed to protecting Superman, that could still potentially change — especially with Clark dividing himself and a more unstable Jordan now having superpowers (albeit weaker ones).
Another agent associated with Project 7734 was Benjamin K. Krullen (aka Reactron). Though created separate from the project, he was subsequently recruited. Zealous, he waged war on Kryptonians in Metropolis and their home cities. He even killed some prominent figures, including Supergirl's father: Zor-El. No matter what transpires with Lane, the mere fact that Captain Luthor is trying to forge alliances could pave the way for Krullen's appearance. More tellingly is the fact that Luthor's method of war has been through building armored suits. Krullen's abilities came from such equipment known as the StarSuit. Furthermore, Luthor has had a vested interest in nuclear facilities. That was mostly to test Superman. However, those very tests could lead to imbuing a suit with the exact Nuclear and radiation-based powers that Krullen exhibited as Reactron.
A version of Reactron appeared previously in Supergirl season 1, episode 3, "Fight or Flight". Played by Chris Browning, he had a longstanding grudge against Superman. This Reactron was ultimately defeated by Kara. Since Crisis on Infinite Earths, however, the Arrowverse has been established upon an entirely new timeline. As such, the shared continuity has the freedom to craft new origin stories (even for previously established villains). Equally, characters could be recast with no questions asked. After all, in Superman & Lois alone, Wolé Parks is able to play a new Lex without disrupting Jon Cryer's version and Sam Lane was switched from being played by Glenn Morshower to Dylan Walsh. As such, Reactron could just as easily emerge with a new face. And give Superman (and Lois) even more of a rogue's gallery to deal with across the remaining episodes.