Ymir's character could be developed by exploring her past more deeply. For example, her time as a Titan before joining the Survey Corps. This could make her more complex as she struggles with her identity. Christa, on the other hand, might develop by breaking out of her 'nice girl' persona. As she faces more challenges with Ymir by her side, she could become stronger and more assertive.
In some fanfictions, the bond between Eren and Ymir is shown as a fateful one. Eren, being a key figure in the fight against Titans, is often seen as being drawn to Ymir's power and the secrets she holds. It can be a very intense connection, like an unspoken understanding that they are both part of something much larger in the Attack on Titan universe.
One possible fanfiction could be about their adventures together within the walls. Maybe they discover a hidden passage that could potentially change the fate of the Survey Corps. They might face Titans together and in the process, their bond deepens as they rely on each other's skills and instincts.
Well, Ymir was born from the void in Norse lore. Ymir's body was used to create the world as we know it in the mythology. For example, the mountains were made from her bones, the seas from her blood. Her back story is crucial as it sets the foundation for the entire Norse cosmological view.
One key element is definitely Ymir's emergence from the chaos of Ginnungagap. Then there's the significance of his body being used to form the world. His bones for mountains, his flesh for land, all these are important elements that show how the Norse believed the world was created from this one giant being. It also implies a certain hierarchy in the cosmos as the later - arriving gods were the ones who shaped the world from Ymir.