Well, without having read it, it could potentially center around a transformation where a non - human entity becomes human just for one night. It could explore how this new human form affects their interactions with other characters, their understanding of the human world, and perhaps there are some adventures or romances that occur during this one - night - only human experience.
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.
Well, without more information about this 'cause youre the only one 3 fanfic', it's difficult to give a definite answer. You might want to join some fan communities related to the original work (whatever it is). People in those communities often share and discuss great fanfics, and they may be able to point you in the right direction for this particular one.
I'm not sure as I haven't read it specifically. But generally, a love novel might have themes like the journey of falling in love, the challenges faced by the couple, and the growth of the relationship.
Not having read the 'cause youre the only one 3 fanfic', it's impossible to say the plot precisely. But if we think about fanfics in general, they can be about many things. Some fanfics focus on character development, showing how a character grows or changes in a new situation. Others might be about exploring relationships between characters in a deeper way. There are also fanfics that introduce new elements or concepts to the existing world of the original work. So, the plot of this particular fanfic could be any of these or something completely different.