A cool concept would be a joint mission. The Supernatural side has to deal with a powerful, ancient evil that has some connection to the vampire origin in the TVD world. So, they team up. Dean might be initially hesitant about working with vampires like Stefan and Damon, but they soon realize they need each other's skills to defeat this new threat.
One idea could be that a vampire from TVD ends up in Riverdale. Maybe Damon Salvatore finds himself in the small town for some mysterious reason. He could get involved with the mystery-solving that the Riverdale gang is always up to. He might use his vampire charm and powers to help or sometimes complicate things. Another plot could be that a character from Riverdale discovers the existence of vampires and werewolves like in TVD. For example, Jughead could be intrigued by these supernatural beings and start writing a story about them, which then draws the attention of the TVD characters.
One idea could be that the characters from TVD end up in the PLL world due to a magical mishap. For example, Damon and Stefan could find themselves in Rosewood and get involved in the mystery of 'A'. They might use their vampire skills to protect the PLL girls from the unknown threats.
The key is to find common ground. For example, both shows deal with the supernatural. So, a plot where a powerful supernatural entity threatens both the world of Supernatural and Mystic Falls would bring the characters together. Sam's knowledge of lore could combine with Bonnie's magic skills to fight this entity.
A really interesting plot could involve the Tesseract from the Avengers. It could be that the Tesseract has some sort of connection to the Darkness in SPN. When the Avengers are trying to protect the Tesseract, they accidentally release some of the Darkness. The Winchesters, with their knowledge of the supernatural, and the Avengers, with their superpowers, must then join forces to contain the Darkness again. The characters could also have some really fun interactions, like Dean being in awe of Iron Man's technology and Steve Rogers being impressed with Sam and Dean's hunting skills.
Perhaps a fanfiction where an ancient spell gone wrong results in multiple characters believing they are mates with one person. This would cause jealousy, confusion, and lots of power struggles as they all vie for the attention of this one individual. For example, Damon, Stefan, and Klaus all thinking they are mated to Elena in some strange new supernatural way.
One popular theme is the interaction between characters from different shows within the crossover. For example, characters from 'The Vampire Diaries' might meet characters from 'The Originals' and have to work together or face off against a common enemy. Another theme could be the exploration of different magical elements. Since both shows have a lot of magic involved, fanfiction might delve deeper into how the magic systems interact. Also, romantic relationships between characters from the different shows can be a big draw. Fans often like to imagine their favorite characters from each show getting together in unique and interesting ways.
Well, a plot could revolve around the father of the baby. He might be in a difficult situation, like being trapped in some sort of supernatural bargain that endangers the pregnancy. Or perhaps the pregnant character forms an unexpected alliance with a former foe because they both have an interest in the well - being of the unborn child. It could add a lot of drama and new dynamics to the fanfiction.
One idea could be that they face challenges from the vampire and werewolf communities. Since their relationship was already complex in the show, this pregnancy could stir up more trouble. Another plot could be them trying to keep the pregnancy a secret while preparing for the arrival of the baby. They might have to deal with issues like finding a safe place to raise the child in a world full of supernatural threats.
Well, a possible plot is that Elena's pregnancy is the result of a spell gone wrong. Klaus, despite his initial shock, decides to stand by her. This could lead to a series of events where they have to fight off those who see the baby as a threat. It could also explore how Elena deals with the physical and emotional changes of pregnancy while being in a rather strange relationship with Klaus. And the baby might have some unique abilities that are both a blessing and a curse.
Well, one interesting plot point is the clash of the different vampire abilities. In Twilight, vampires have certain powers like super speed and strength, and in TVD, vampires have different powers like compulsion. When combined in fanfiction, seeing how these powers interact or conflict can be really exciting. For example, if a Twilight vampire tries to use their speed against a TVD vampire who can compel them, it creates a whole new dynamic.