there is a small island
But if it's a formless void how can it be there? I demand physics and mathematical equations to support your description. Seems wayyy too loose without them. Go on 15 page mathematical calculations please.
and when the civilization's origin became tied up in the gates,
Was of course kidding about the last one, but this one seems like it could use a little more expansion...unless you've got plans for later.
spanned light-years
Interesting. That usually isn't done in these stories.
assigned guardians to each of the planets inside its borders
Again going into some detail on the others and offering some hypothesis on their origin and then we don't get any for the Senshi? Odd omission.
and prevented her from throwing herself into a futile death, all so the nine of them would have a chance for a future.
Nice enough summary as it goes with all this prior information but as all kind of explanations such as this are it's kind of impersonal. Since you're already doing a time travel thing with Usa or whatever why not use these same lines but adapt them to dialogue and have your character equivalent of Usa be learning about this in a class maybe? That'd maybe seem more interesting and also make this more personal while also allowing you to interject some personality into the story.
and send them
and sent them
Venus' deception
Venus's deception. Grammar rule is almost always an 's even if it is after a name that ends in s.
silver millenium
Capitalized? Not sure.
“O-okay. What do I need to know?”
Interesting characterization.
But Luna doesn't have enough time.”
Expand that. Since obviously that's what she's doing, simply say that is what you're doing buying time.
future me, what have you done?”
Ehh...could be better. Make this more personal. She's watching herself die. This should be pretty disturbing for her.
the Moon Princess again,
Seems a bit impersonal really, could use something more.
Sailor Saturn had been awakened
Mistress 9 possession is not a problem?
that idea was vetoed by Sailor Saturn herself; she refused to end everything as long as there was a chance to defeat the Dark Kingdom without it.
Think you need to show that scene. Seems important.
Not a bad start at all. Could use a bit more expansion and a bit more showing of scenes in a few places but overall a solid start. I do have a couple questions though. As to the Senshi themselves have you thought about how having Ranma as a Princess and no Sailor Moon up till later then in canon is going to change them? Usagi was sort of the glue holding them together as they are a rather disparate bunch. I'd imagine Ami to be a lot more reclusive, Makoto to be a bit more violent, Rei to be a lot more standoffish, Minako to be forced into a more traditional leadership role, Hotaru to have a lot more confidence and not fear her power since she is saving the world right now, Haruka and Michiru might be pretty unchanged but will likely respect a less flighty leader a bit more so their canon personalities will be tempered, and Setsuna is going to be radically changed what with being forced into a leadership role and also because they'll likely look up to her solely due to the age difference. Anyway, again intriguing start look forward to seeing where you take it.