Another theme might be transformation or the blurring of boundaries between human and snake. Maybe the snake girlfriend has the ability to transform into a human - like form, and this creates interesting plot points about identity and acceptance.
In 'ash girlfriend fanfic', a prevalent theme is the power of love. It shows how Ash's relationship with his girlfriend gives him extra motivation in his Pokémon battles. Also, the theme of discovery is common. Discovering new feelings for each other, as well as new Pokémon and places together. There's also the theme of loyalty, both in his relationship with his girlfriend and with his Pokémon.
One common theme is trust. Since Snake and Emma are likely from different backgrounds in the fanfic world, they need to build trust. For example, in many stories, Snake initially doubts Emma's intentions but as the story progresses, they learn to trust each other.
One common theme is empowerment. Female Solid Snake is often depicted as a strong, independent character who can hold her own in the dangerous world of the Metal Gear series. Another theme is identity, as she may struggle with being different in a male - dominated world.
Well, teamwork is a big theme. They are both skilled fighters, and in fanfics, they combine their abilities to take on powerful enemies. Also, redemption can be a theme. For example, if Raiden has made mistakes in the past, Snake might help him find a path to redemption. There's also the theme of mutual respect. Despite their differences, they come to respect each other's strengths in many fanfics.
A common theme is the clash and then the fusion of their different worlds. Samus comes from a world of space exploration and alien battles, while Snake is from a world of military espionage and political intrigue. Their stories often involve them finding common ground.
I'm not really into this kind of fanfic, but generally, it might involve the unique characteristics of snakes in a fictional and often very imaginative way. Maybe it could be about the relationship between snake - like creatures and other beings in a made - up world.
One common theme is the struggle for power. The OC snake shinobi often has to fight against other powerful characters in the shinobi world to gain recognition or protect something important.
The theme of protection also pops up a lot. The snake might protect the human from other threats in their environment, or the human might protect the snake from those who fear it. This creates a bond between them that is based on mutual trust and reliance, which is a central part of many 'human x snake fanfic' stories.
Love and devotion are common themes. The boyfriend's willingness to risk everything to save his girlfriend shows the depth of his love. Another theme is heroism. He becomes the hero in the story, whether it's saving her from physical danger or from emotional distress like in some stories where he saves her from depression.
Jealousy is a very common theme. The possessive girlfriend gets jealous easily, whether it's about Naruto's female friends or his time spent on training. Another theme is over - protection. She wants to protect Naruto from any perceived threats, real or not.