Another theme is growth. Fanfictions such as 'Growing Together' show how Ted and Rebecca help each other grow personally and professionally. They learn from each other's experiences and become better versions of themselves, which is really inspiring.
Another theme is personal growth. Both Ted and Rebecca have their own issues to deal with, and fanfictions often show how they grow as individuals while being part of each other's lives. For example, Ted's homesickness and Rebecca's past relationship traumas are often explored in relation to their relationship.
A theme that often pops up is redemption. Maybe Nathaniel has made mistakes in the past and through his relationship with Rebecca, he tries to redeem himself. And sometimes, the fanfictions explore the theme of family, how their families (or lack thereof in some cases) influence their relationship and how they build their own little 'family' unit together.
One common theme is the discovery of self - through the relationship. Billy and Rebecca often learn more about themselves as they get to know each other better.
Overcoming hardships is also a popular theme. Since both characters have their own struggles in the original story, fanfiction might focus on how they help each other get through difficult times. For example, Asta's lack of magic initially and Rebecca's family situation could be elements that are intertwined in stories about them supporting each other through tough times.
One common theme is adventure. Since Luffy is an adventurer, the fanfictions often involve them going on quests or exploring new places together. Another theme is friendship. Their relationship usually starts as a friendship which then may develop into something more.
In many yugi x rebecca fanfictions, the theme of family also plays a role. Rebecca's family connections and Yugi's family - like bond with his friends can influence their relationship. For example, how Rebecca's family views Yugi and vice versa can add an interesting layer to the story.
One common theme is love against all odds. In many fanfictions, there are obstacles like family differences or social status disparities that Rebecca and Billy have to overcome to be together.
One common theme is friendship. Their meeting often leads to the start of a strong bond between them. For example, in many stories, they start off as strangers but quickly become friends through shared experiences.
One common theme is adventure. Often, they are put in exciting situations together, like exploring new planets or facing off against alien threats. This allows for a lot of action and character development.
One great Ted and Rebecca fanfiction is 'The Unspoken Bond' on FanFiction.net. It beautifully explores their relationship's nuances.