Well, first of all, Archive of Our Own is a top destination. It's known for its extensive library of fanfiction, and the tagging system makes it easy to find what you're looking for. You can also try Tumblr. There are often posts about fanfiction, and some users might share or recommend Merlin Roxy fanfictions. And don't forget about Reddit. There are subreddits dedicated to fanfiction, and you might be able to find some great Merlin Roxy stories by asking around or searching through the posts.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Merlin and Roxy. Just use the search bar and type in their names.
A common theme is the exploration of their relationship. It could be a slow - burn romance where they gradually fall in love. Another theme might be them facing challenges together, like battling against a common enemy in a magical realm.
There are often fanfictions that explore their relationship in a more in - depth way. For example, how they first met and how their friendship developed over time. Some might even add a bit of a romantic undertone to their relationship, like a slow - burn kind of love story.
One great Merlin Roxy fanfiction could be 'The Adventures of Merlin and Roxy'. It might focus on their journey together in a magical realm, facing challenges side by side. Another could be 'Merlin and Roxy: A Bond Forged in Magic' which could explore how they first meet and start to trust each other. And 'Merlin, Roxy and the Enchanted Forest' might be about their exploration of a mysterious forest and the secrets they uncover there.
Another option is to look on social media platforms. Some groups on Facebook or Reddit dedicated to Batman might have members who have written or can recommend Batman and Roxy Rocket fanfiction. Just be aware that the content might not be as organized as on the dedicated fanfiction websites.
I'm not sure of specific Merlin x Roxy fanfiction titles off the top of my head. But you can try searching on fanfiction.net as well. You might find some interesting takes on their relationship. Maybe there will be ones where Merlin uses his magic to protect Roxy in a dangerous situation, which could lead to deeper feelings between them.
One popular theme could be adventures. Maybe Roxy Morton and Merlin go on a quest together to save their world from a dark magic threat. Another theme might be friendship, showing how their different backgrounds and skills complement each other as they face various challenges. And the theme of discovery could also be present, like they discover new magical elements or secrets about each other's pasts.
I think Gaius would be an excellent character to include. His wisdom and knowledge of magic could be really useful for Roxy Morton and Merlin. Then there's Lancelot. His chivalry and noble nature would fit well in the story. Also, Nimueh. She has her own power and backstory that could create some intense plotlines when paired with Roxy Morton and Merlin.
Another place to look is FanFiction.net. It has a wide range of fanfics from different fandoms. Just type in 'roxy x john' in the search bar, and it should show you the relevant fanfics if there are any.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and there's a good chance you'll find Vivian Merlin fanfiction there.