The Ultimen team from "Ultimatum" represents a "reunion" or fusion of various previous versions of the Justice League. The Ultimen themselves are based on characters created for Super Friends (1973) who were not previously existing comic book characters. Shifter and Downpour are based on the Wonder Twins, Long Shadow is based on Apache Chief, Wind Dragon is based on Samurai, while Juice is based on Black Vulcan. Additionally, the structure atop the high rise, which is the Ultimen's headquarters is modeled after the iconic Hall of Justice. The character of Max Lord is the same as the comics character introduced in the 1990s who took over the post-Crisis Justice League and remodeled it as an international business enterprise with loose ties to the United Nations. Furthermore, the Justice League members who oppose the Ultimen in the episode are those who were best known as the "Super Friends": Batman, Superman, Aquaman and Wonder Woman.
"The Squad" Waller mentions calling in is The Suicide Squad.
According to Dwayne McDuffie, "the beast that threw itself against the bars" is a nod to the Wonder Dog from the 1970s Super Friends cartoon.
Wind Dragon calls Batman "old chum," which is what Batman used to call Robin on the Batman series from the 60's.