Another theme is the exploration of their differences. Shigaraki represents chaos and destruction while Deku represents hope and heroism. Fanfics usually explore how these two opposing forces interact and influence each other. For example, in some fanfics, they start to understand each other's perspectives better as they spend more time together.
To write their relationship realistically, you need to consider their personalities. Deku is kind - hearted and determined, while Shigaraki is more cynical and vengeful. So their interactions would be full of tension. For example, if they have an argument, Deku might try to reason with Shigaraki using his ideals, but Shigaraki would likely mock or reject them.
One interesting plot could be that Deku and Shigaraki, as brothers, are on opposite sides at first. But then they discover a common enemy that forces them to work together. This would involve a lot of character development as they have to overcome their differences. For example, Deku's belief in heroism and Shigaraki's hatred for the hero society.
One great 'Shigaraki x Deku' fanfic is 'Unlikely Bonds' which explores a unique friendship-turned-something -more between them. It delves deep into their complex characters and how they might interact in different scenarios outside of the main plotline.
Shigaraki is not dead in the manga. The storyline has many twists and turns, and his character plays a significant role, so his survival adds to the ongoing drama and suspense.