One plot idea could be Lee and Carley surviving together in a new, previously unexplored area. They find an abandoned farm with resources but also face new threats like a group of bandits. Another could be a story where they get separated from the main group and have to rely solely on each other to make it back.
Their relationship could develop through mutual trust. As they face various dangers together, they rely on each other more and more, which deepens their bond. For example, when one is injured, the other takes care of them without hesitation.
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.