Tips For Making Better Sailor Moon OCs

First, some stuff about star seeds and sailor mechanics.

Star seeds are actually a pretty important part of the workings of the Sailor Moon universe, but there's a dearth of information on how they work on the Internet. So here's a quick primer to give you some idea of how star seeds work.

• In most Sailor Moon continua, all forms of life (and even stars, moons, planets, and asteroids) carry star seeds within them, which are in essence their souls. For the sailor senshi, it's also the ultimate source of their power. While the star seeds of non-senshi lose their shine (IE, die) after the death of its carrier, the star seeds of the senshi (referred to as "sailor crystals" in the manga and "true star seeds" in the anime) are immortal, and so as long as their star seeds are not destroyed they will be reincarnated. In the manga, all star seeds come from the Galaxy Cauldron, which is located in the center of the galaxy. Sailor crystals are sent out to become the guardians and protectors of celestial objects.
• Moar star seed shenanigans: single star seeds don't seem to necessarily be linked to the same celestial body for eternity. Both Usagi and Chibi-Usa are Sailor Moon, and Chibi-Usa definitely has a sailor crystal unique from Usagi's. In the manga, Sailor Cosmos appears and is implied to be a future incarnation of Usagi.
• So far, celestial objects that have definitely had guardians are planets, planetoids, moons, asteroids, the galaxy, and even the universe itself. The Sailor Stars imply stars having their own senshi, though it's never explicitly stated that this is the case. There have never been senshi associated with entire constellations, and we've seen a lot of senshi in the franchise. So a Sailor Denebola might not be out of place, but a Sailor Leo would bit a bit odd.
• Men can carry sailor crystals - in the manga, Tuxedo Kamen is the carrier of the Earth crystal. However, rather than having an outfit with a sailor collar, his outfit is a white tie evening suit with a cape and top hat. He even has his own attack - Tuxedo La Smoking Bomber.
• Another thing to keep in mind - Usagi's Silver Crystal/ginzuishou is not her starseed. Nothing can live with its star seed removed from its body, and Usagi has been shown getting along just fine and dandy without the silver crystal being in her body. In the manga, Sailor Galaxia stole Mamoru's star seed by pulling his Golden Crystal from his body in the shape of a lotus, which opened to reveal his star seed inside of it, indicating that the starseed can hide inside (and merge with?) other crystals, though.

Other senshi stuff!

• Sailor senshi are frequently referred to as having "the protection of their guardian planets," implying that some of their power derives from their associated celestial objects as well as their own star seeds.
• Many senshi have two powers or themes to base their powers on. For example, Sailor Pluto has death and time, Sailor Saturn has death and silence, Sailor Venus has love and metal, and Sailor Jupiter has wood and storms. Bearing this in mind, you could combine multiple themes/elements yourself to put a more unique style or twist on a common element or to give yourself more room for variety in your character's attacks and techniques. Also, if you're having trouble thinking up things to base your character's powers on, then the Magical Guardian Element Generator might help you come up with something.
• The size of the celestial object a sailor senshi is associated with does not necessarily indicate strength of power! Sailor Moon is canonically one of the most powerful senshi in existence, yet the moon is a fairly small object. There are four senshi (the Sailor Quartet) associated with asteroids in the manga, and none of them are depicted as weak.
• While most of the senshi from our solar system were princesses of their respective planets in the manga, being a senshi does not automatically mean being royalty or reincarnated royalty, so don't be afraid to think outside that box. That said, if you really want or need your character to be royal, check out Tips To Write Better Royalty, Nobility, & Other Upper-Class & Important Characters.
• Animal companions are not inherent to the senshi package, either - your character (or your character's team) need not necessarily have one. They aren't necessary to discovering that one is a senshi, after all - while Luna kept hold of Sailor Moon's transformation brooch after her death on the moon and gave it to her after she was reborn on Earth, there have been instances of senshi awakening and transforming on their own. For example, in the Infinity arc of the manga a reborn Setsuna awakens and transforms into Sailor Pluto, and in the anime Haruka's transformation pen simply appeared before her. If you have a good reason to give your senshi an animal companion, that's fine - but don't feel obligatated to do it Just Because.
• Figure out your OC is supposed to do as a senshi. Is your character supposed to protect someone, like the Sailor Starlights and the Sailor Quartet protect their respective princesses? Or does your OC guard something, like Sailor Pluto guards the Space-Time Door? Is your character supposed keep a guard post, such as how Sailor Uranus and Sailor Neptune are supposed to look out for and guard the solar system against external threats? Or is you senshi appointed with some other job or task?
• However, don't assign your senshi to do things that already have people doing them just fine. Sailor Moon has four very competant sailor senshi plus Tuxedo Kamen to protect her. Chibi Moon has the Sailor Quartet. Pluto's done just fine guarding the Space-Time Door thus far. All of them have allies they can easily call upon if they need an extra hand. None of them need a new character out of nowhere suddenly assigned to help them.
• If you plan to do a Sailor Earth or Sailor Sun, tread carefully. First, both of these concepts have been done many, many times already, so you won't look too original with them. Secondly, Mamoru carries Earth's sailor crystal in the manga, so if you're doing a Sailor Earth you may need to do a little work to avoid any major plotholes there.

Senshi outfit design!

• A couple of things to remember about sailor fuku design: First, while all sailor senshi had sailor collars, only the senshi from our solar system were shown to wear the schoolgirl-style uniform. Everyone else had very different designs. Secondly, sailor senshi who are affiliated with each other usually wear similar outfits. There may be minor variations between them (EG, Sailor Mercury's lack of sleeves, Sailor Saturn's chest brooch, and Sailor Mars's waist gem; or Sailor Phi's pantsuit and Sailor Chi's skirt), but overall their outfits will look similar enough that you can tell they belong together just by looking at them.
• If your character is supposed to be part of a canon team, before designing your character's outfit take a good look at the outfits of the team your character is supposed to be with. Which aspects of their costumes remain the same on each and every one of them, excepting the leader? Keep those elements the same on your own character's outfit as well.
• Avoid borrowing distinctive costume elements from senshi your character is supposed to have no affiliation with. For example, if your character isn't supposed to have any connection to the Sailor Starlights, don't make your character's brooch look like theirs.
• Unless your character is a future or past Sailor Moon, or has some profound connection to Sailor Moon, odango are best avoided. Likewise, avoid giving your character a hairstyle that is very obviously a canon character's hairdo with a length and/or color change.
• If you're having trouble coming up with designs for an extrasolar sailor senshi, the Super Special Sailor Suit Generator might help spark your imagination. Also, for general tips on designing your character's costume and look, check out Tips 'N Stuff For Better Character Design.

And other things!

• Your character doesn't necessarily have to be a senshi to potentially kick butt. In canon works we've seen witches, scientists, aliens, fairies, a character who basically amounted to an ice/snow elemental, and even a vampire. While these types of characters have served as antagonists, there's no rule stating that they absolutely have to.
• If your OC is supposed to be from an extrasolar planet, why not do a little worldbuilding and figure out what your character's world is like and how it's different from Earth? Coming into contact with unfamiliar Earth customs could create difficulty for your character, or start opportunities for conversations - EG, "On my world, we don't have those..." "Oh? What do you have?"
• Basic Tips To Improve Your OCs & Fan Characters has more general OC creation tips.