To those who haven't read the original novel, in the novel's 20th chapter, the novel's protagonist, Parzival along with two Japanese brothers, Shoto and Daito took a quest in Planet Tokusatsu called "Shodai Ultraman" in which the trio recreate all 39 episodes of the original Ultraman TV series as members of the Science Patrol. The reward for said quest is Hayata's Beta Capsule, allowing the user to transform into Ultraman for three minutes. Said item played a big role in the novel's final battle.
Other robots that you can select as a reward are: Gigantor/Tetsujin 28-go, Tranzor Z/Mazinger Z, Jet Jaguar, Giant Robo, the entire Shogun Warriors line (notable robots part of the line are: Daitetsujin 17, Combattler V, Daimos, Danguard Ace, Getter Robo, Gaiking, Grendizer, Great Mazinger, Raideen and Voltes V), and the mechs from the Gundam (RX-78-2 Gundam was seen in the trailer) and Macross franchises.
Ready Player One is set for March 30, 2018 release.