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.
I don't think Darren Modok was featured in the comics. Usually, characters like this don't just pop up without notice or a lot of buzz among comic fans.