Well, it's hard to say for sure. Sometimes a Touhou comic might stick closely to certain expectations, while in other cases it could take some creative liberties and deviate.
It might be a story of cultural exchange in the Touhou universe. Wriggle comes from a more bug - centric area and Cirno from a cold, icy realm. The fanfic could explore how they learn from each other's cultures, with Wriggle teaching Cirno about the importance of bugs in the ecosystem and Cirno showing Wriggle the beauty of ice sculptures and how to skate on ice.
One common theme could be friendship exploration. Since they are both characters from a certain context, their relationship might be depicted as evolving from acquaintance to a deeper bond in the fanfic.
The setting of their world. Their world is full of magic and mystery. A fanfic can explore different parts of this world that we haven't seen much in the original works. It could take the readers on a journey through the dark forests where Wriggle lives among the bugs, or to the places where Mystia sings her enchanting songs. This exploration of the setting can make the fanfic really engaging.
In my opinion, the story of Flandre Scarlet is also a great one in the Touhou series. Flandre is a powerful vampire who has been locked away for a long time. Her story is about her isolation, her struggle with her own power, and her attempts to interact with the outside world. It shows her complex personality, from her child - like innocence to her terrifying strength when provoked. This contrast makes her story very engaging.
Well, start by familiarizing yourself with the Touhou characters and their backstories. Then, plan out a simple storyline that fits the Touhou universe.
Well, it could be about their adventures in the magical world they inhabit. Maybe they team up to face a common threat, like an evil spirit that is disturbing the peace of their land. They could use their unique abilities - Wriggle's power over bugs and Mystia's singing skills - to overcome challenges.
You can start by looking on some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of Touhou - related fanfictions, and there's a good chance you'll find some Mystia x Wriggle stories there. Just use the search function and type in the relevant tags.
One great Mystia x Wriggle fanfiction could be a story where they team up for a music event in Gensokyo. Mystia, with her love for singing, and Wriggle, with her unique bug - related abilities, might create an interesting dynamic. They could face challenges like dealing with a noisy audience or a rival music group, and in the process, their friendship could turn into something more romantic.
One plot could be a competition. They might compete in a magical contest where Wriggle uses her bug - related skills and Cirno her ice powers. For example, they could be competing to see who can build the most impressive structure in a short amount of time, with Wriggle creating a huge bug - made tower and Cirno an elaborate ice castle.