Danny Phantom is a U.S. animated television series that lasted from 2004 to 2007.
The version of Earth in this canon is directly linked to a Ghost Zone, where the spirits of Earth's departed reside. The two worlds are separated, but linked, and ghosts frequently return to the living side, usually for causing mischief and mayhem in their former home.
Danny Fenton's parents are smart but bumbling researchers who construct a portal (the Fenton Portal) into the Ghost Zone. Danny was wandering around in their lab one day and accidentally activated the portal while standing inside. His DNA became infused with ectoplasm, making him half-living human and half-ghost (or "halfa," as many of the ghosts call him).
Danny gained the ability to change his appearance into a ghostly one, in which he has ghostly superpowers and works to stop the hauntings occurring around Amity Park, capturing ghosts in his parents' Fenton thermos and bringing them back to the Ghost Zone. Danny gained many recurring enemies along the way, most notably Vlad Masters, his parents' former partner who developed powers similar to Danny's, and hates Danny and his father in particular.
Danny Phantom fanfiction sees plenty of shipping, as well as Mary Sues from both sides of the Fenton Portal. The fact that ghosts figure prominently in this canon gives characters the opportunity for trajeck backstories. (Although many canon ghosts do have plots related to their living selves, the most frequent recurring villains apart from Vlad are presented as just being evil ghosts, without needing any excuse in their past to make them bad guys.)
Romantic pairings in this canon can be further complicated by ghostly possession of other characters (which can also include plain old Sue-wraith possession as well). Sam Manson (one of Danny's best friends and eventual Love Interest) and Dani Phantom (a younger, female clone of Danny made by Vlad) have a very high rate of being Sued, especially in romance fics. Danny himself can display Mary-Suish tendencies in romance fics or during combat scenes. Fics also sometimes make Danny out to be Vlad's son, since they have powers in common and because Vlad, despite being evil, is much cooler to preteens than Danny's bumbling but loving parents are.
One inhabitant of the Ghost Zone, Clockwork, seems to have the power of time travel and awareness of alternate universes in canon. He may not snap back with canon in fics that involve him, and require direct neuralyztion.