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.
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.
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.