One idea could be an OC who is a skilled medic. They could travel between different communities in the Kingdom, facing challenges like zombie attacks and distrust from some groups while trying to save lives. This OC could also have a backstory related to the pre - apocalypse medical field, which adds depth to their character.
Think about their pre - apocalypse profession. For example, if they were a teacher, they might be good at teaching survival skills to others in the Kingdom. Their personality could be patient and nurturing, but also strong - willed as they've had to adapt to the new world. Give them a distinct physical feature, like a scar from a past zombie encounter that they got while saving someone.
Michonne does die in the comics. The circumstances surrounding her death were quite dramatic and had a significant impact on the story's progression. It happened during a major conflict with other characters.