They could start with a mutual respect for each other's survival skills. For example, if the male OC is good at scavenging and Maggie is great at leading, they recognize the value in each other. This respect can then slowly turn into a deeper bond as they face more challenges together.
One idea could be that the male OC is a former enemy who has a change of heart and ends up saving Maggie from a dangerous situation. Maybe he has some unique skills, like being an expert tracker, which helps them in their fight against the walkers.
One common theme is leadership. Carl shows potential for leadership from a young age, and Maggie has experience leading. Fanfictions might explore how they work together to lead the group, make decisions about where to go and what to do. Also, the theme of hope often comes up. Despite the dire situation, they keep looking for a better future and inspire others in the group to do the same.
There are indeed. Fanfiction writers often explore different aspects of their relationship. It could be about their sisterly love, how they remember their family before the apocalypse, or their dreams for the future in that harsh world. By looking for fanfictions that are tagged with positive themes like 'sisterhood' or 'survival', you can avoid the inappropriate ones and find great stories about Beth and Maggie.
In the comics, unfortunately, Maggie meets her end. It's a sad moment for fans. The circumstances of her death add to the complexity and drama of the story.