I think an important note to make for people who either want to write fan fiction or do a roleplay of Sailor Moon that I have seen a lot of people miss is the importance of the names.

Probably it is the fault of the American dub where in an attempt to Americanize everything the characters were pretty thoughtlessly given random western-sounding names that well... perhaps one can forgive "Raye", "Amy" and "Mina" since they really aren't all that different.

But the thing is that their names mean a lot more if one reads kanji.

Tsukino - Of the Moon
Usagi - Rabbit

So Tsukino Usagi is "The Rabbit of the Moon", which is what Japanese people say they see when they look at the shadowy parts of the moon.

Hino - Of Fire (using the same first kanji as the Japanese name for Mars)
Rei - Spirit

Kino - Of Life (using the same first kanji for the Japanese name for Jupiter)
Makoto - Truth

Mizuno - Of Water (using the same first kanji for the Japanese name for Mercury)
Ami - Wisdom

Aino - Of Love (and this one disrupts the pattern because she was thought of first. Otherwise her name would have been Kinno which would have been confusing)
Minako - Minako

A similar pattern continues on throughout the Outer Senshi, only deviating a little, but not by much. In fact, except for Hotaru, every one of the other's last names is the same as the planet only without the "no" at the end like the Inner Senshi. One could then certainly imagine that any senshi past or future that would be part of her court born as humans would probably have names that fit that particular pattern.

Similarly we have
Chiba - Earth Field
Mamoru - Guardian


We can pretty much surmise that his guardians ought to have the same sort of name pattern reborn as humans. Although the Shintennou were never given names for their human forms and it seems virtually every version of the series ever produced bent over backwards to avoid doing this, it would probably not be difficult to figure their names would probably have the second kanji of Chiba.

So, for instance, if one figures that Shintennou are the the four directions in Buddhism, we could probably figure the first kanji ought to be the direction.

Kitaba - Northern Field
Touba - Eastern Field
Nishiba - Western Field
Nanba - Southern Field

And none of these names would be particularly unusual to boot. To go further, if we were going to stick to the first initial of the names of their gemstones (J, K, N and Z) we could probably come up with decent names that also contain elements to suggest their personalities. Z doesn't really work, but in Japanese Z and S are pretty close.

Jun(ichiro) - Purity
Nobu(ro) - Spirit
Satoru - Intelligence
Katsu(o) - Victory

If there were any other Earth guardians out there (and since all four cardinal directions are covered, it would be difficult to tell how it would work) one can imagine they would adhere to a similar pattern.

Also, typically guardians come in sets of 4.
Which means whereever you see a set of 3 in the series, there is probably a good slot for an original character to appear without disrupting anything. A fourth member of the male Amazons or a fourth Kinmoku senshi wouldn't be out of the question (although in the former case, it disrupts the group name).

Wow, this was really cool to read!  Thanks for posting this!  I'll make sure to remember this next time I am thinking of a fanfic or roleplay idea

You know, following my thoughts about how the Shittennou probably ought to be named as humans, as opposed to all the noncanon names or aliases they have had such as "Izuo Saitou" or "Kun Saitou" or "J. Daitou"...

Does it really even seem likely that the guardians for Prince Endymion were named after random gemstones? Yes, when killed they turned into rocks, assumably the rocks they were named after, but...

Well, look. In both canon and noncanon material they have never been referred to by any other names even in scenes that took place in the age of the Silver Millenium and Golden Elysium, and they didn't have any other names even after dying and being turned into ghosts, but...

What if they were developed out more fully?
Random gemstone names seem to be a staple of Sailor Moon villains. You got a villain? Give it a random gemstone name. No one with a random gemstone name is ever a hero, always a villain and always it seems as though they have their power derived from Chaos.
Basically... a random gemstone name seems more likely to be what they were named by darkness rather than being their true name.

Well... what other names could they use? What would by their equivalent of "Endymion"?

Well, the term Shittenou as I noted comes from the Bhuddist term for the four directions.
They are called
North: Tamonten - Associated with the color yellow as well as the umbrella or the mongoose. He hears everything.
South: Zouchouten - Associated with the color blue and the sword. He causes things to grow.
East: Jikokuten - Associated with the color white and the pipa (a guitar like instrument). He upholds the realm.
West: Koumokuten - Associated with the color red, associated with the serpent or pearl. He sees all.

Given where the Shitennou are said to have their domains
Jadeite - Asia
Nephrite - North America
Kunzite - Middle East
Zoisite - Europe

I think we can kind of line them up and see that neither their colors nor personalities line up.
But, maybe the issue is that.. well... none of the other names are from Japanese mythology.
Endymion? Selene? Those names are Greek/Roman. (The romans never changed the names.)
Mercury, Venus, Mars, Jupiter, Uranus, Saturn, Neptune, Pluto? Those names are all Roman.

So what are the Greek/Roman equivalents to the Shitennou? Well... the Four Winds.
North - Aquillo 
South - Auster 
East - Vulturnus
West - Favonius

Not much to be said about their personalities. Basically they were associated with bad weather and otherwise the typical tyrannical kidnappers and rapists as all Greek deities more or less were. But one never much needs to pay much attention to these details.

Interestingly enough though, if one were to go by the Four Winds, one could have a set of Outer Shittenou.
Northeast - Caecius
Southeast - Subsolanus (He actually replaces the East wind in Roman mythology, trading groups with Vulturnus)
Northwest - Caurus
Southwest - Africus

One could even perhaps consider lining them up with the continents in the southern hemisphere - South America, Africa, Australia and Antarctica. You can tell which one is associated with Africa quite easily.

I just figured I'd offer these ideas up in case anyone wants to use them.