How about a Deltarune and Pokémon crossover? The Lightners could be Pokémon trainers in a strange new region. Ralsei might befriend some Pokémon that have a connection to his own magical abilities. And Susie could be a tough trainer who uses her strength to battle other trainers. There could be a big mystery involving Darkners and some rare Pokémon.
One idea could be a Deltarune and Undertale crossover. You could have the characters from Deltarune end up in the Underground from Undertale. They might interact with Sans and Papyrus, for example. The contrast between the two worlds' rules and the different character personalities would create a lot of interesting scenarios.
A Deltarune and Steven Universe crossover could be cool. Maybe Kris and Susie find themselves on Beach City. They could meet the Crystal Gems and have to work together to solve a problem that involves both the magic in Deltarune and the Gem powers. The Deltarune characters' unique abilities would be interesting to see in the context of the Steven Universe world.
One idea could be that the characters from Undertale find themselves in the world of Deltarune. For example, Frisk could end up in the Dark World and have to figure out how to get back home while interacting with the Deltarune characters. They might team up with Kris to fight against new threats that are a combination of both universes' villains.
One idea could be that Danny Phantom gets sucked into the Deltarune world. He could use his ghost powers to help the characters in their quests. Maybe he teams up with Kris to fight against the dark forces in the Dark World.
One interesting idea could be a fanfic that focuses on the backstories of the minor characters in Deltarune. For example, exploring what the lives of the Darkners were like before the events of the game.
One idea could be that the characters from Undertale find themselves in the Deltarune world and have to adapt to its different rules and characters. For example, Sans could meet Ralsei and they could have a humorous exchange about the nature of their respective universes.
Maybe in the fanfiction, the Dark Fountains from Deltarune start appearing in the Undertale world. This causes chaos as the monsters in Undertale don't understand what these strange fountains are. Frisk, along with the Deltarune trio, has to figure out how to stop the fountains from spreading darkness. They could learn from each other's powers and strategies. For example, Frisk could teach Kris about determination in a more in - depth way, and Kris could show Frisk how to navigate the strange new areas created by the Dark Fountains.
One interesting 'Deltarune' fanfiction could be the ones that explore the backstories of the characters more in - depth. For example, there might be fanfictions focusing on Kris's inner thoughts and struggles. Another type could be those that create alternate universes where the game's events take a different turn, like if the Darkners and Lightners had a more peaceful co - existence from the start.
One interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
Another plot idea is a story where the antagonists from both games, such as Flowey from Undertale and the King from Deltarune, form an alliance. Their goal could be to take over both universes. This would force the heroes of both games to work together. The fanfic could explore how the different characters with their own powers and personalities interact and strategize to defeat this powerful alliance.
A Rambo crossover fanfiction could involve him crossing over into the Star Wars universe. Rambo's guerilla warfare tactics could be interesting in a galaxy far, far away. Maybe he helps the Rebel Alliance in a ground - based battle against the Empire. Or a crossover with the Predator series. Rambo would be a worthy opponent for the Predator, and the fight between them in the jungle could be epic. Also, a Rambo and John Wick crossover would be intense, two highly skilled fighters on the same side or perhaps initially at odds.
One idea could be a crossover with a modern military setting. The Decepticons could be trying to steal advanced military technology. Another is a crossover with a post - apocalyptic world where they are competing with other factions for resources. A third could be a crossover with a magical realm, with the Decepticons trying to harness the magic for their own evil purposes.