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.
Their relationship could become more protective. Rowan would be even more vigilant about Aelin's safety and well - being, constantly hovering around her. And Aelin might become more dependent on Rowan in some ways, relying on his strength and support during her pregnancy.
Rowan could be one of the most interesting. His loyalty to Aelin and his own strength and backstory add depth. He might be on a quest to save her, and his actions and decisions can be really engaging. Then there's Lysandra. Her shapeshifting abilities and her complex relationship with Aelin make her a fascinating character. She could be using her skills in unique ways to help Aelin in the fanfiction.
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.
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.
The plot also matters. It should be engaging, perhaps with new challenges for them to face as a couple. Like a new magical threat that they have to overcome together, which tests their relationship and makes it stronger. It could also explore their relationship in different settings or time periods that we haven't seen much in the original stories.
Love is a very common theme. Their passionate love for each other is often explored in great detail, from the initial attraction to the deep and abiding love that develops over time.
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.