The idea of Sailor Earth isn't a bad one to me but the idea of Naru being Sailor Earth irks me.
I honestly have no idea why people want her to be a Senshi at all.
Aside from the whole idea that Serena abandons Molly and her other friends and writers want to recapture that dynamic, there is a plot basis for this trope.
During the Neflyte episodes, at least in the dub, it's hinted that there's something special about Molly. Note that Molly was not only willing to put her life on the line for him when she thought he was an innocent (which is a heroic trait), but she was willing to believe in Neflyte even though she knew he was a liar. At this point in Sailor Moon, Molly had more compassion than the titular character.
Thus, turning Molly into a Sailor Sc0ut, while cliche, is an extension of her development in the early episodes. Could you imagine the potential for drama if Molly had turned out to be another Sailor Sc0ut, especially if a planetary symbol appeared on Molly's head at the pivotal moment when she steps in front of Neflyte? Instead of Sailor Moon's tiara magically stopping on Sailor Moon's command, what if this revelation caused it to stop...either via a burst of willpower, or if Neflyte saved her...if only to be used as a pawn. As an alternative, what if Molly had been the Moon Princess all along?
I like to think Usagi didn't completely ditch Naru but they simply drifted apart once Naru stared dating Umino.
So Umino has kinda served as a compensation for her.
Except that really doesn't work because Melvin is not only Serena's friend, but her classmate.
Back to the original topic, I did not turn Molly into Sailor Earth, but I did something similar for her. If you read the summary of my discarded fanfic
, I made her into a Defender. A Defender is my fan-term for characters who aren't Sailor Sc0uts but have powers. Since they weren't representing a planetary body, like Tuxedo Mask* they wore masks and were temporary figures of fighting evil. In my fic idea, she took Sailor Moon's place when the Sailor Sc0uts were defeated, and Molly did have control over the earth element.
However, my stupidest idea for a fan Sailor Sc0ut was Sailor Andromeda. I don't mean the Sailor Andromeda which was a one-shot idea where Ann posed as Sailor Sc0ut
. That was bad enough. No, in my vision of Sailor Moon, Queen Serenity's mother, Queen Luna, was romanced by the Demetrite (Alan and Ann's race) Fian, and it was expected that the courtship would turn into marriage, as this would unite the Moon Kingdom and the Demetrites. However, Luna decided to marry a commoner instead, but she gave Fian a "seed of power" as a goodbye present, and this seed, when it reacted with a woman, she'd become Sailor Andromeda.
Aliana, a daughter of the Tree of Life (which had settled on a new planet), came to Earth, and unknown to even herself, was chosen to be Sailor Andromeda. Unfortunately, she got possessed before she found out herself and consequently, she fought against the Sailor Sc0uts until freed of the influence. Then like Alan and Ann before her, she left to make no impact on the story again. Despite this, I made a fake bio. I even used Snake Man's theme as her theme.
*Yes, I know that Tuxedo Mask is connected to Earth, but in my fanfic he's not a male guardian of Earth as much as he was a defender.