In certain comic arcs, Heimdall tragically meets his end. The circumstances and reasons for his death can vary depending on the specific comic series and plot.
Perhaps Sakura is amused by Itachi's unique skills or his way of handling things. Meanwhile, Shisui annoys her because he has a different approach to things that she doesn't like. In the fanfiction, it could be that Shisui is too brash compared to Itachi's calmness, and Sakura, being a fan of Itachi, just can't stand Shisui's antics.
Another theme is the exploration of power dynamics. Heimdall has his own powers and responsibilities, and when paired with an OC, it can create an interesting dynamic. Sometimes the OC might be in awe of Heimdall's power, or they might challenge him, leading to a more complex relationship.
In Heimdall x Reader fanfiction, a common theme is the struggle against fate. Heimdall may know the reader's fate, and the reader may try to change it, which creates conflict and drama. Another theme is the idea of seeing and being seen. Heimdall's all - seeing ability makes this a central aspect of the relationship. He sees the reader in a way no one else can, and the reader may strive to understand him in return. Additionally, the theme of sacrifice can be present. Heimdall might have to sacrifice something for the reader, or vice versa, as their relationship deepens.
Perhaps it's their disobedience. They never listen to the adults or follow the rules in the story. This causes chaos and trouble for the other characters. For instance, if they are told not to touch something precious and they do it anyway just out of defiance.