Toward the end of The Forever People, DC editorial forced Kirby to include Deadman as a supporting character.
The Hunger Dogs. After being hired to give an end to the story of the New Gods but being forbidden to write the ending he had planned (with Orion and Darkseid dying, Kirby turned in a 24 page story called "On The Road To Armaghetto". The story (Orion goes to Apokolips to stop turncoat New God Esak from creating a new doomsday device for Darkseid) effectively gave no ending whatsoever to the storyline and was rejected). A deal was then struck to give Kirby a 64 page graphic novel to wrap up his story, but on top of the "No killing Orion and Darkseid" edict, DC also ordered Desaad, Kalibak, and Steppenwolf resurrected as they were to be included in the Super Powers toyline that was being released. Further complicating things was the decision by DC to make Kirby rework and incorporate the already finished pages of "On the Road To Armaghetto" into the graphic novel and then rearranged the page order, against Kirby's will.
Some of this is disputed by Mark Evanier in his afterword to Jack Kirby's Fourth World Omnibus Vol. 4. According to him, it was Kirby that didn't want to bring Darkseid and Orion Back for the Dead.
Reality Subtext: Mister Miracle and Big Barda's relationship is based on Jack's relationship with his beloved wife Roz. Some have even suggested that the entirety of the New Gods were a metaphor for the Cold War and the Vietnam War.
Technology Marches On: Averted considering Jack Kirby was visionary enough to create fantasy technology that often felt ahead of its time. The most obvious example are the Mother Boxes, handheld artificially intelligent computers that usually look like small computer tablets without a display screen. Vykin of the Forever People carries a larger boxlike version, but that is more for ease of handling considering the team must lay their hands on it to exchange places with the Infinite Man.
Reportedly Jack Kirby originally conceived the entire series for the Marvel universe when he still worked at Marvel before his falling out with Stan Lee - more specifically he envisioned the New Gods replacing The Mighty Thor and all his fellow Asgardians, who he intended to perish in Ragnarok or some other similar catastrophe, and there are some very strong similarities (perhaps most notably being Highfather's similarity to Odin). This becomes Hilarious in Hindsight considering that Marvel's Thanos was based on Kirby's Darkseid.
Writing by the Seat of Your Pants: The way Mark Evanier tells it, Kirby had a plan for his series...it's just that the details were liable to change at a moments notice.