The relationship between Robby and Miguel is complex in the original source material. Fanfiction allows exploring their relationship further. It could be the idea of them finding common ground or resolving their differences in ways not shown in the official story.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). There are often tags related to 'Robby x Miguel' which can help you filter the stories. Many writers share their works there, and you might discover some great fanfictions.
One common theme is friendship. They often start as friends or rivals and then develop a deeper bond. For example, in many fanfics, they first compete in some activity and then realize they have more in common than they thought.
A great 'tory x robby fanfic' that I've come across is 'The Hidden Feelings'. It starts with a rather tense situation between them and then gradually unfolds their true emotions for each other in a very engaging manner.
Friendship is a very common theme. Many fanfics explore how Marco and Miguel become friends, what they do to maintain their friendship, and how it withstands various challenges.
The exploration of the afterlife itself is a theme. Miguel and Hector could go on adventures within the Land of the Dead, uncovering secrets and learning more about the nature of the afterlife together. This could involve meeting other spirits, solving mysteries, and growing closer as they do so.
There's also 'Miguel and Hiro's Adventure'. In this fanfic, they team up to solve a mystery in their city. Miguel's street - smart skills and Hiro's technological genius complement each other, leading to an exciting and action - packed journey.
Another theme is self - discovery. As they interact, both Miguel and Hiro often discover new aspects of themselves. For example, Miguel might find a new interest in technology through Hiro, and Hiro might become more street - savvy because of Miguel.
One common theme is the exploration of family. Since Miguel has his own family issues and Ernesto has his past family - like relationship with Hector, their interactions can often be used to explore family values.