Makima's fate in the manga is quite complex and involves a series of dramatic events. She faces powerful opponents and has her own agenda that leads to significant developments.
One common theme is the exploration of their complex relationship. Given Makima's manipulative nature and Denji's naivete in the original story, fanfictions often delve into how their relationship could develop in different scenarios. Another theme could be redemption, perhaps for Makima's actions towards Denji, and how they could move forward from that.
Often, there is a theme of an unlikely alliance or relationship. Given that they are from different fictional universes, the fanfic might focus on how they come to meet and form a bond. This could involve a lot of world - building to bridge the gap between their respective stories.
One of the common themes is trust - building. Given Makima's nature, the reader in the fanfic may initially be hesitant to trust her. So, the story could revolve around how they gradually build trust. Also, the theme of sacrifice can be present. Maybe the reader has to sacrifice something for Makima or vice - versa in the course of the story. There are also fanfics that focus on the theme of redemption, where Makima or the reader might seek redemption through their relationship.
Perhaps there could be a story where the father of the baby is a mystery, and Makima is on a journey to figure out who it is. This could involve her investigating past relationships or encounters in a world full of secrets and power plays.