One possible theme could be the exploration of identity. A male Sanghili and a male human in a fanfic might grapple with their own identities within their respective species while also forging a unique bond. Another theme could be the power dynamics, as the Sanghili are often depicted as a strong and proud species, and how that plays out in their relationship with a human in the story.
Since I'm not aware of many such fanfics, it's hard to say. But it could potentially explore themes of friendship across species, cooperation in a shared adventure, or perhaps cultural exchange between the two characters.
I'm not sure specifically about male Sanghili and male human fanfic. Fanfics can be quite niche and vary widely in availability. You could try searching on specialized fanfic websites or forums dedicated to the relevant universes to see if any exist.
Often, the theme of discovery is present. The human discovers the alien's world and vice versa. It can also be about love against all odds. Their relationship might be frowned upon by their own species, but they still fight for their love. And there's also the theme of learning and growth as they adapt to each other.
One common theme could be the exploration of an unusual relationship in a science - fiction context. Maybe the idea of two very different species interacting in ways that are not typical.
One common theme is cultural exchange. Since the two species are so different, exploring how they learn about each other's cultures is a big part. For example, a male human might be fascinated by the yautja's hunting traditions.
A theme that frequently shows up is the protection of the weak. Toriel is a very motherly figure, and in these fanfics, she often extends that protectiveness to the male human. Also, there's the theme of cultural exchange. Since the human is from the surface world and Toriel from the Underground, they share their different ways of life. For example, the human might introduce Toriel to things like human music or art, while Toriel teaches the human about monster magic or the Underground's history. This exchange helps to strengthen their bond.
The exploration of power is also a theme. The male human may gain some unexpected power or have to deal with the power dynamics between the different factions. For example, he could be caught in the middle of the feud between the vampires and werewolves because of his unique position or abilities.
One common theme could be the power dynamic. Since the female is an elite, there might be exploration of how the male human deals with her status and power. Another could be a kind of 'fish out of water' scenario if the male is from a more ordinary background.
One common theme could be adventure. The human male and female furry characters might go on an adventure together, facing various challenges. Another theme could be friendship that gradually develops into something more.
Love and acceptance are quite common themes. Despite their differences, the male human and the female monster fall in love. This shows that love can transcend physical and cultural differences. And acceptance comes in as they both have to accept each other's flaws and differences to make the relationship work.
Romance is also a very common theme. The idea of a human male and an elf falling in love despite the odds. It can be a slow - burn romance, where they gradually get to know each other over time. Or it can be a more sudden, passionate connection that defies the expectations of their respective societies.