One common theme is the connection between the living and the dead. Dia de las Muertos is about commemorating the dead, and in fantasy novels, this often takes the form of characters being able to communicate or interact with their deceased loved ones.
Perhaps it could center around a family that has been cursed. On Dia de las Muertas, the veil between the worlds is thinnest, and they have a chance to break the curse. They encounter various helpful and hindering spirits during their quest. The novel could explore themes of family, heritage, and the power of love and remembrance as they strive to end the curse.
One great 'dia de las muertos fantasy novel' is 'The Day of the Dead' by various authors. It weaves together the rich cultural elements of Dia de los Muertos with a touch of magic and mystery.
A young artist. He could be using his art to connect with the spirits on Dia de las Muertas. His paintings might come to life, and he becomes involved in a magical adventure in the spirit realm to save his village from a dark force that is awakened during the celebration.
One option is to start with a strong hook. It could be a startling statement such as 'The day the robots took over, no one was prepared.' or 'In a future where time travel is possible, the first mistake has been made.' This type of opening makes the reader curious and eager to find out what happens next. You can also start with a description of a future society's norm that is very different from our own. For instance, in a world where people are born with the ability to teleport, but there are strict laws governing its use.
A good way is to start with a vision of the future. Paint a picture of a world where technology has advanced in a particular way, like flying cars are as common as bicycles today. This gives the reader a sense of the setting right away.
Since I'm not familiar with the 'Monster Dia and Meg story', it could potentially be a story about friendship between two monsters. Maybe Dia is big and strong while Meg is small and clever. They might go on a journey to save their monster homeland from some kind of threat, like an evil wizard or a natural disaster that is endangering their kind. However, this is all just speculation without more context.
Friendship is a very common theme. Since Chika and Dia have a strong bond, fanfics often explore different aspects of their friendship, like how they support each other through tough times.