Borrowing heavily from the Batman Beyond episode "The Call," the opening sequence for the test footage utilizes mostly animation from the episode's elaborate climatic sequence, where buildings in Metropolis are exploding for reasons unknown. However, this sequence also borrows from "The Call" in a sneakier, subtler way: the debut of the Justice League directly mirrors the moment where the Justice League Unlimited emerges from the smoke and prepares to take care of business. Take a look:
Emerging from the clouds first, in both versions, is Superman, striking a similar pose.
Next to materialize are the Hawk-related characters; Hawkgirl in the pilot and Warhawk in "The Call." The water-related character (Aquagirl in the JLU) is omitted in the Justice League test footage, however, as Aquaman is not part of the team.
Third to emerge from the smoke is the Green Lantern, surrounded by the energies of the power ring, who exits the frame towards the right.
Following the prior characters are character's that fill the female warrior slot: Wonder Woman (Justice League) and Barda (JLU).
The similarities end here, of course, as there is no J'onn J'onzz equivalent character in the JLU. However, it is fascinating to note that the borrowing from previous shows doesn't stop with the backgrounds and select animated scenes, as this sequence pays homage to the first Justice League-related team that the creative team had animated.