One of the main themes is grief. The characters in the bardo are all dealing with their own forms of loss and unfulfilled desires. Another theme is the idea of transition and the afterlife. It explores what might happen to souls in the in - between state. And also, it touches on the theme of father - son relationship, as Lincoln's relationship with his son is a significant part of the story.
Well, the themes include the exploration of the human condition. In 'Lincoln in the Bardo', we see characters in a sort of limbo, and this reflects on how humans face uncertainty and change. The historical aspect of it also brings out themes of war and its impact on society. And of course, the relationship between the living and the dead is a significant theme, as Lincoln is mourning his son while the other characters in the bardo are in their own state of waiting and reflection.
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.
The 'meg and dia monster short story' could potentially be about Meg and Dia's adventure in a world where monsters exist. Maybe they have to face a particular monster and overcome challenges related to it, like finding a way to defeat it or escape from it. It could also be about how they discover the existence of the monster and how it changes their perspective on their world.