Radiant Historia had an interesting team of developers working on it. The game was originally conceived by Tri-Ace’s Satoshi Takayashiki, concept designer on Radiata Stories, who pitched the title to Atlus because he was fond of their games.
This was revealed in a developer Q&A you can read on the game’s official website.
Takayashiki felt that Radiant Historia would benefit from Atlus’ experience developing role-playing games. Once the pitch was improved, sure enough, Atlus, as the game’s publisher, requested certain changes be made to the concept design. The first was a radical change to the game’s protagonist.
The protagonist of Radiant Historia was originally to be a sword. The story would revolve around those who came in possession of the weapon. Atlus argued, however, that players wouldn’t be able to relate to a protagonist that wasn’t human, and so the protagonist was changed to Stocke, who also ultimately underwent several changes.
The second change involved the game’s timeline. Radiant Historia was originally going to have two separate axes — horizontal and vertical. Lines on the vertical axis would represent different timelines, and the horizontal axis would present the game’s story, which was originally going to be a historical drama. This was changed to the simpler timeline the game presently uses.
The third change was to the game’s ending. Initially, Radiant Historia was going to have a tragic end where everything would end in ruin. This was changed as well. Radiant Historia now offers multiple endings, with one “true” ending. Finally, while the game’s battle system had already been chalked out, Atlus were the ones responsible for the order in which skills were acquired and how they were balanced.
As far as influences go, Takayashiki drew inspiration from fantasy novels, as well as other franchises that have detailed settings and characters, such as One Piece, Gundam, Warhammer and Dragon Quest.