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In a recent interview, Nasu discusses the development of FGO. He stated that the scenario was completed up to Chapter 7 before release. After receiving initial fan feedback after release, he decided to make it more "Fate" in which he rewrote chapters 5-7. https://ddnavi.com/interview/653186/a
Nasu took inspiration from Arthurian romance and "Makai Tenshou," a novel taking place during the Edo period where legendary swordsmen get revived. He drafted a novel with a modern twist of a heroine that summons King Arthur to fight in Shinjuku. He later shelved that novel.
Other works that inspired Nasu included Yukito Ayatsuji's "The Decagon House Murders," Hajime Kanzaka's "Slayers," and a novel spinoff of PC game "The Screamer." Various works by Hideyuki Kikuchi inspired him such as "Demon City," "Vampire Hunter D," and "Alien."
Dubbed "Old Fate," it was Nasu's third full-length novel he had written who was a high schooler at that time. He stopped writing the novel up to the part where King Arthur fights Sasaki Koujirou at the gate of Ryuudou Temple.
Takeuchi was aspiring to be a mangaka, & Nasu was discouraged because he felt novels were no match for manga regarding certain action scenes. Then he read "The Decagon House Murders" & was inspired by its presentation of mystery. This led him to creating "Kara no Kyoukai."
After the creation of Tsukihime, Nasu wanted his next work to exceed expectations. On a train ride back home from Comiket, Takeuchi suggested bringing back Old Fate, but to turn it into a game with King Arthur as a girl. Nasu commented how his head was spinning at that time.
Takeuchi's brother was also interested in seeing more Old Fate. However, Nasu didn't want to write another boy meets girl fantasy so he said if they are going to do it, it's going to be 3 times more interesting than manga of that genre. Thus Old Fate turned into Fate/Stay Night.
When UBW got a TV anime, Aniplex producer Shizuka Kurosaki said she wanted to read a story of the Round Table. Nasu notes that those who only saw UBW wouldn't know of Artoria's backstory, so he wrote Garden of Avalon which focused on Artoria's past life, & was bundled with the BD
Garden of Avalon was the basis for FGO Chapter 6, as Nasu was able to write more about the Knight of the Round Table there where he couldn't in the short novel. Thus, their story was 30 years in the making. From Old Fate, to Fate/Stay Night, to Garden of Avalon, to finally FGO.
While writing Chapter 6, Nasu saw Maaya Sakamoto performing "Replica" live and was inspired to have the final fight the atmosphere of the song. As such, Bedivere's final battle become more that of a "radiant story" thanks to the power of Maaya Sakamoto's song.
With Chapter 6, Nasu wanted wanted players to feel a stronger sense of immersion by overcoming hardships so he suggested more powerful enemies. He was met with opposition from some of the staff, but convinced them this would allow players who overcame them to grow with FGO.
When Camelot was getting a 2-part film, Nasu was worried how all of it would fit in 3 hours, admitting there will be a lot cuts. He then saw the stage play version which was 2 hours. Due to the great performance, he was convinced an anime could work as a drama focused on Bedi.
Nasu closes the interview praising the voice cast and staff working on the Camelot films.
