Naru was just a character written to give Usagi someone to interact with before all the Guardians joined the cast. The anime remembered her for a bit, but she was largely reduced to a minor role because the story just didn't need her anymore. I highly doubt she was ever written to be anything more.
Naru’s sole function was to just be Usagi’s friend before the Senshi started appearing and to give Usagi someone to save that she has a personal stake in the first act. Her role is severely diminished after Act 1. The anime treated her a lot better keeping her around a bit longer. It helped that there was 7 episodes with just Usagi as a Senshi and making Usagi and Rei at odds with each other gave Usagi more reason to hang out with Naru even with Rei and Ami being her other friends but once Mako shows up you could slowly see the anime staff phase Naru out that after Minako shows up Naru only appears in about 3 episodes in the rest of the Dark Kingdom series.
Frankly, the idea Miss Takeuchi considered Naru to be the Moon Princess or Sailor Saturn is laughable