Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
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.