Mamoru abuses Usagi or dies. Usagi runs to Seiya/Demand/Haruka. 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 Seiya/Demand/Haruka.
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.
Alternatively, the story focuses on Chibiusa in the 30th Century, all grown up and being a badass, with a senshi team either consisting of the canon (in the manga, anyway) Sailor Quartet (formerly Amazoness Quartet), original characters meant to be the original senshi's daughters, or both.
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.
The most popular Sailor Moon fanfic villain in the 90s that wasn't just an OC was Jadeite, coming back from his eternal sleep from the original anime and trying to get revenge for his undignified end. Even if the story wasn't explicitly about his return, there was always a good chance a story's OC villains would discover Jadeite somehow, break him out of that crystal prison, and turn him loose on the Senshi. These have finally died off in the decades since, but it was such a common plot point as to be the subject of parody in the early Sailor Moon fandom days.
The Shitennou are reborn in present-day identities following the conclusion of the Dark Kingdom arc and rediscover their SilMil-era relationships with Mamoru and the Inner Senshi. Teaming up to fight a new threat and/or wrestling with guilt and mistrust over their stints as villains both optional.
Kunzite mourning the loss of his love, Zoisite. These can range from simple angsty one-shots to longer plots (Kunzite working with the Senshi to bring Zoisite back to life, turning to Usagi for comfort, betraying Beryl).
Nephrite comes back from the dead to rekindle his romance with Naru, or accepts that she's moved on. Either way, he keeps the chocolate parfait promise.
If they didn't want to break Haruka and Michiru up but still wanted to purge homosexuality from the show, the story would instead be about how Haruka was reborn as the wrong gender and Queen Serenity would "fix" it by turning her back into a boy. Gained particular notoriety when a prominent fan organization, Save Our Sailors, attempted to pass off their ownFix Fic as actual canon in the late 90s. Yea, don't do this either.
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.
Rei is secretly in love with Usagi! Unfortunately, she's destined for Mamoru, so Rei must suffer valiantly in her unrequited love or wait for Mamoru to either nobly step aside, turn into a dick or get run over by a truck so she can make her move. Though one fic has Mamoru totally cool with Usagi dumping him for Rei and giving her his sperm in a can to make ChibiUsa. No, really.
Chibiusa and Helios meeting again in the 30th Century.
Conversely, depending on the writer's opinion of Sailor Pluto, Usagi kicking Pluto out for not being a planet (which makes little sense since she's, you know, Sailor Moon), Usagi vowing to restore Pluto's planetary status when she's queen, or Pluto leaving to join and/or head the Kuiper Belt Senshi.
Sailor Pluto using time travel to help another character correct a mistake in their past... except it will always been something mundane on the cosmic scale like helping one of the characters with their relationships. Often without any actual justification that the event Pluto is helping them fix will actually impact the time line (in the series, Pluto is only seen intervening on apocalyptic level events and appears rather fatalistic about things like her own death.)