Another option for the main antagonist could be a group of hunters. They are after both Danny Phantom because he is a half - ghost and Medusa because of her powers. They see them both as threats to the human world and will stop at nothing to capture or destroy them. They might have advanced weapons and technology specifically designed to combat ghosts and mythical creatures.
One idea could be that Danny Phantom accidentally enters Medusa's lair while on a mission in the Ghost Zone. Medusa, being intrigued by this half - ghost, half - human, decides to use his unique abilities for her own gain. Another plot could be a crossover where Danny has to team up with Medusa to fight a common enemy that threatens both the human world and Medusa's domain. Or perhaps Medusa gets ahold of Danny's ghost powers and goes on a rampage in the human world, and Danny has to figure out how to stop her without destroying her.
In a fanfiction, Maddie could be working on a project that involves Lemon. Danny, being curious, sneaks in to check it out. This leads to a series of events where they all have to work together to solve a mystery. For example, the mystery could be about a strange energy source that is affecting the town and Lemon has some knowledge about it from her own experiences.
In most cases, if the fanfiction is not being sold or used in a way that directly competes with the original work, it's usually okay to read. But if the lemon content involves violating any laws such as those related to obscenity, then it's not legal regardless of it being fanfiction. Also, different countries have different laws regarding these matters.
One idea could be a story where Danny and Ember have to team up to defeat a new ghostly threat that is causing chaos in Amity Park. This could involve Ember using her music powers in new ways and Danny having to rely on her more than he expected.
His betrayal would likely be the catalyst for a whole new set of story arcs. Other characters might start to question their own loyalties. And since it's a 'lemon' fanfiction, there could be a sense of bitterness that spreads throughout the story. Maybe the town that Danny protects would also start to turn on him, thinking he can't be trusted anymore. It would be a downward spiral for Danny's character, at least initially, as he has to deal with the consequences of his actions.
A possible plot could be that Maddie is trying to create a device that can communicate with ghosts in a more peaceful way. Danny, being half - ghost, is both excited and a bit worried about this. Lemon, who has a more open - minded view on the supernatural, encourages Maddie. Danny then gets involved in the project, and they face many challenges along the way. For example, some other ghosts might not want humans to be able to communicate with them so easily, and they try to stop the project. This could lead to a story full of drama, with Danny having to use his powers to protect Maddie and Lemon while also trying to support Maddie's scientific goals.
One great story could be 'The Sibling Bond' where Danny and Jazz face a new ghost threat together. In this fic, their relationship is explored deeper as they rely on each other in the face of danger.
There are some fanfiction platforms like Archive of Our Own (AO3). It has a large collection of fanfictions, but make sure to follow the site's rules and guidelines. Also, FanFiction.net might have some relevant works, though you need to search carefully using the right keywords related to Danny Phantom.