Perhaps a crossover where the magic systems cross over. The valg from 'tog' could somehow seep into the world of 'acotar' and the characters from 'acotar' have to learn about the valg and how to combat it using the knowledge that Aelin and her crew have from 'tog'. This could lead to some really interesting battles and alliances.
One idea could be to have the characters from 'Throne of Glass' (TOG) and 'A Court of Thorns and Roses' (ACOTAR) meet in a magical realm created by a powerful ancient artifact. For example, Aelin from TOG could be on a quest to find a way to defeat a new, powerful enemy and accidentally stumbles into the world of ACOTAR. There, she meets Feyre and they initially have a bit of a clash due to their different upbringings and leadership styles, but then realize they can work together to solve a problem that threatens both of their worlds.
When writing an ACOTAR and TOG crossover fanfic, character development is key. You can't just throw the characters together without considering how they will grow and change in this new situation. For instance, if you pair up Chaol from TOG with Lucien from ACOTAR, think about how their different backgrounds and experiences will influence their relationship. Maybe Chaol's loyalty to his kingdom will be challenged by Lucien's more fluid view of loyalty among the fae. Also, don't be afraid to experiment with the magic systems. Try to find unique ways for the characters to use their combined powers. This will make your fanfic more engaging and original.
Good fanfic writing for a TOG and ACOTAR crossover requires a deep understanding of the source materials. Research the lore, the politics, and the relationships in both series. For the plot, think of something that challenges the characters from both worlds. A good way is to introduce a new, powerful enemy that neither world has faced before. In terms of character development, let the characters grow and change as a result of their interactions. For example, if Rowan from TOG meets Azriel from ACOTAR, they could learn from each other's combat skills and ways of thinking. Also, pay attention to the pacing of your story. Don't rush the important moments and build up the tension gradually.
First, re - read both series to get a firm grasp on the characters, their personalities, and the worlds they inhabit. Then, decide on a central plot point that would bring the two worlds together. It could be as simple as a portal opening between the two realms. Next, choose which characters from each series will be your main focus. For example, you might want to start with Feyre and Celaena as they are strong female leads.
Maybe a character travels forward in time and sees a dystopian version of the ACOTAR world. They then have to go back and try to prevent the events that led to that future. They could face many challenges along the way, like not being believed or having to work against powerful forces who benefit from the future they saw. It would be a race against time to save their world.
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.
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.
One idea could be a crossover with Naruto. Kakuzu could be sent to the Naruto universe and form an unlikely alliance with a shinobi village to fight a common enemy, like the Akatsuki from his own world turned rogue in the new universe. This would create a lot of interesting character dynamics and battles.
Another option is a crossover with 'The Little Mermaid'. The characters from the descendants could meet Ariel and her friends underwater. Evie might be interested in the mermaid fashion and could design new outfits inspired by the underwater world.