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.
One great plot twist could be that Shigaraki has a hidden noble intention behind his actions. Maybe he's trying to expose a deeper corruption in the hero society that no one else sees. It makes readers re - evaluate his character.
In certain fanfictions, Shigaraki's redemption is tied to his past. Maybe he discovers some long - forgotten memories that make him understand his true self and that he doesn't have to be a villain. He then starts to use his power for good instead of destruction.
One common theme is the exploration of morality. Meruem's own moral compass is complex, and in crossovers, it gets further explored when he interacts with different characters from other universes. Also, friendship or rivalry is a big theme. He either forms new alliances or makes new enemies in these crossovers, which drives the story forward. For instance, in some fanfictions, he has to decide whether to trust a character from another world or not, which adds a lot of depth to the story.
One great 'Meruem x Reader Fanfiction' is 'A Meeting of Minds'. It really delves into the complex relationship between Meruem and the reader. The way it describes their interactions, from the initial curiosity to a deeper emotional connection, is really engaging.