A common theme could be friendship. Since Ness and Lucas are both from the same game universe, their relationship can be explored in terms of a deepening friendship.
Perhaps a fanfic where Ness is on a journey to a new land and stumbles upon Lucas who is in trouble. Ness helps Lucas out and from there, they embark on a journey together. Along the way, they learn about each other's cultures and the unique aspects of their homes.
One common theme could be friendship. Given the nature of these characters, fanfics might explore how Ness, Lucas, Ninten, and Claus form deep bonds with each other. Another theme could be adventure, as they all come from a world full of strange creatures and phenomena in Earthbound. They might embark on new adventures together.
One common theme is self - discovery. Brooke often goes on a journey to find out more about herself. Another theme is friendship. She forms strong bonds with other characters in many fanfics. And love is also a common theme, whether it's romantic love or love for family and friends.
A frequent theme is the exploration of their future. Writers of Eliott and Lucas fanfics often imagine what their life will be like years down the line. This includes things like career choices, living together, and maybe even starting a family. Another common theme is dealing with external pressures, like society's views on their relationship, and how they stand strong against it.
Another theme is growth. Lucas starts out as a novice trainer in many fanfics. As he travels and battles, he becomes more skilled and knowledgeable. He learns about different Pokemon types and how to strategize better in battles. This growth theme is really important as it gives the story a sense of progression.
One idea could be a story where Ness, Lucas, Ninten and Claus are all brought together by a strange psychic phenomenon. They have to team up to figure out what's causing it and how to stop it. Along the way, they share their different experiences from their respective adventures and learn from each other.
One place to start looking for 'ness x lucas fanfic' could be on fanfiction.net. They have a large collection of fan - created stories, and you might be able to find some interesting ones there.
One common theme is friendship development. Since they are often friends in the original source material, fanfics might explore how their friendship deepens over time. Another theme could be adventure. They might go on some sort of adventure together and face challenges that bring them closer.
One common theme is the bittersweet feeling of leaving school. It could be about saying goodbye to friends and teachers. Another theme might be the anticipation of the future, like what Brooke and Lucas will do after graduation.
Often, friendship - turned - love is a theme. Since they might start as friends in the original context, in fanfic it can be interesting to see how that relationship evolves into something more romantic. Also, themes of loyalty and support can be present, as they face challenges together in the stories.