So I made up a storyline for Buffy the Vampire Slayer and then the series ended and I still have this plot in my mind so here it is:
The story opens in a year sometime before season seven of Buffy the Vampire Slayer.
While slaying some demons who are messing about with a time amulet Buffy disappears to be replaced by a different slayer. It turns out the time amulet exists in three different eras where three different slayers have interrupted the evil plotting of demons who are up to no good with all the amulet magic and with the chanting.
One of the three slayers is called Greta and lives in medieval Europe, the second is Buffy in Sunnydale and the third is called Deva and lives about 200 years in the future when humans have faster then light drive and colonise the galaxy. Bufy disappears and finds herself in the future. Deva, the slayer from the future is displaced into medieval times and Greta is displaced from medieval times into the Sunnydale of Buffy's time. They fight demons in each other's worlds and then work a spell to return. However the spell doesn't work properly the first time and Buffy goes to medieval Europe while Greta goes to the future and Deva to the present. They fight some more demons and eventually all get home.
When Buffy is in the future everything seems very Star Trek-like except that the aliens are well aware of demons and hell dimensions. There are many, many slayers at work in the future but it is still never enough because there's the entire galaxy to patrol for the slayage.