I think that Chibiusa is Sailor Earth.
It's been hinted that Endymion is not the representation of the earth, but of the sun. I mean it makes sense because Helios is also the name of the god Helios who is the personification of the sun. It also makes sense because Mamoru's birthday is August 3rd, so what astrological sign does that make him? A Leo, and what planet is Leo ruled by? The sun. It can't just be a coincidence because the rest of the senshi's zodiac signs are conveniently the ones that represent their planet. And not to mention, the Golden Crystal? Gold is usually the color that represents the sun as does silver with the moon.

So back to the main point, Chibiusa being Sailor Earth.

If Mamoru is led by the sun and Usagi is led by the moon and they are both meant to protect the Earth then what if they were to have a child? Wouldn't that child be meant to do exactly the same thing? What kind of Sailor Scout is known as Sailor Mini-Moon? It would be a minor character and Chibiusa is not just a minor character if shes the daughter of two very powerful people.

Just to extend my theory a bit more...

The other senshi have more elemental powers such as, water, fire, electricity, and light then maybe Sailor Earth would also get those powers.

Just a thought!

It's a nice theory. Would certainly make a good story. But there are a few things I'd point out:

I think it's been stated in interviews by Naoko that Mamoru was the representative of Earth, in place of a Senshi. And that the Golden Crystal is Earth's Crystal, which was ruled by the Golden Kingdom, Endymion's kingdom of Earth, during the Silver Millenium (I think Helios stated that during Dream). Plus, if you're going with astrology, Chibiusa's sign is Cancer, which is ruled by the moon (no sign is ruled by the Earth). While she might be known as Sailor Chibi Moon, I think when she grows up, she'll simply be Sailor Moon when the title is passed onto her.

That is true, I always have different headcannons on the same topic so it's a dilemma, haha

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This one is interesting! While it's obvious that she'd obtain Sailor Moon's powers due to the royal bloodline and her own silver crystal, what about her father's powers over earth? Now, I'd like to think she'd inherit some of his abilities-not all of them because moon's power takes priority-like the psychic abilities he has in the manga/crystal; though perhaps not as powerful as his.

I think it's an interesting theory, but I tend to agree with the earlier statement made in this thread that Mamoru has a canon alignment with the Earth rather than the sun.

His surname, Chiba (地場・ちば), provides a clear connection with in kanji: the Earth is chikyuu (地球・ちきゅう). The "地" in both is shared, which follows the trend of characters sharing kanji with their guardian planets.

You are correct that Mamoru's sign Leo is ruled by the Sun, but no zodiac sign has Earth as a ruling planet.

As for his daughter with Usagi being "Mini Moon" even though she is not a background character, within the Sailor Moon storyline Chibi Usa is a supporting character. I love Chibi Usa, personally, but her introduction and initial role in the story is primarily a catalyst for Usagi's development.

That said, I think there's a lot of potential to emphasis Chibi Usa's relationship to the Earth, which (as you noted) both of her parents loved so much and fought to protect. It could make plenty of sense to have her inherit abilities related to this planet—she was born on it, in Crystal Tokyo, and I think an argument could be made for her to inherit both Silver and Golden Crystals. (Just because we only saw her use of the Silver Crystal doesn't mean there's not more that could be explored in fanfic or RP.)

Perhaps she could be a "Sailor Earth" someday, or maybe there's a new title possible that could combine her Moon and Earth background into something else. ♥ Given what we were provided with in the manga and anime, there's room to use imagination when considering Chibi Usa's future.

It would be so cool if Chibiusa was Sailor Earth, or maybe some kind of interesting combination between the two rather than a flat out Sailor Mini Moon or a Sailor Earth.

or maybe something new all together?

Yeah!