Power is a very common theme. Since Yami Marik is a powerful character, many fanfictions explore how he uses his power, gains more power, or loses it. Another common theme is revenge. Given his backstory, it's often a part of the story. For example, he might seek revenge on those who wronged him in the past.
One idea could be a story where Dark Yami and Yugi have to team up to face a new, powerful enemy that neither could defeat alone. They start to understand each other's strengths and weaknesses better during their battles.
The relationship can develop through mutual support during the pregnancy. Yami could be very protective of Yugi, always making sure he's comfortable and has everything he needs. Yugi, in turn, can rely on Yami and show his gratitude, which deepens their bond.
One common theme is the allure of the dark side. Yami Marik is a dark character, and many fanfictions explore how the reader is attracted to his darkness. For example, in some stories, the reader might be initially scared but then gradually becomes fascinated by his power and mystery.
I really like 'Yami Marik's Redemption in Love'. In this fanfiction, the author shows how the reader can gradually change Yami Marik for the better. It has a lot of emotional depth and the interactions between the two characters are very engaging.
One popular trope is the 'power transfer' where Yami Yugi imparts some of his ancient powers or knowledge to Yugi in a fanfic. Another is the 'alternate universe' trope, like a modern-day setting where they are regular high - school students instead of duelists. And the 'bond - strengthening' trope is also common, showing how their relationship deepens through various challenges.
In the manga, Yami and Yugi's relationship is one of mutual support and shared adventures. Yami serves as a powerful ally and a source of strength for Yugi in various challenges.
A common theme is the power of their friendship. Their bond is often explored as something that can overcome great obstacles. Another theme is the mystery surrounding Yami's past and how Yugi helps him to uncover it. Also, the idea of their combined strength in dueling is frequently seen.
Once upon a time, Yugi was exploring an ancient tomb when he accidentally awakened Yami. Yami was drawn to Yugi's kind heart. As they spent more time together, they realized they had a deep connection. Yugi taught Yami about the modern world, and Yami protected Yugi from various dangers. Their love grew gradually, like a flower blooming in a secret garden.