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Fandom Specific Plot:
Mamoru abuses Usagi or dies. Usagi runs to Demand/Haruka/SEIYA. ChibiUsa is either handwaved into a new child or ignored outright.
Alternatively, Mamoru abuses Usagi but she still stays with him for the good of the world and to have ChibiUsa. She then has an affair with Demand/Haruka/Seiya.
Usagi and Seiya's bastard child goes to the past/Earth to learn about her history (and accidentally get her parents back together)/purposely get her parents back together/stop her own birth.
ChibiUsa comes back from the future as a chesty teen and is much more badass.
Usagi is betrayed by Mamoru and/or the Inner senshi. She joins the Outers or Starlights, romances Haruka or Seiya, and is suddenly totally badass/capable/intelligent.
One that was popular in the early days of the English fandom: Amara abuses Michelle. Gary Stu arrives to tell Amara off and rescue poor, vulnerable Michelle from the "sin" of homosexuality. ( Extremely discredited nowadays. Don't write a fanfic like this unless you want to be nuked from orbit by the rest of the fandom.)
Crossovers with where girl-type Ranma joins the Sailor Scouts are common enough to Ranma ½ have their own name.
The Space-Time Door is often written as a magical scrying computer that Sailor Pluto uses to view the past, present and future of many multiple time-lines and universes, often to keep an eye out on "threats to Crystal Tokyo". In the actual series, the door is never shown to be anything but a door.