I'm not aware of any extremely popular 'Yukio visual novels' off the top of my head. There are so many visual novels out there that it's hard to say which ones with 'Yukio' in the title are popular without more research.
I'm not sure specifically as there could be many 'Yukio visual novels' with different themes. It might be about a character named Yukio's story, perhaps his adventures, relationships, or personal growth in a fictional setting.
Hardly. While fanfiction can cover a wide range of themes, abuse is not a typical or popular one, especially when it comes to well - liked characters like Yukio and Rin. The majority of fanfiction writers are interested in creating engaging stories that build on the existing relationships in a positive manner. Depicting abuse would not only be out of character for most interpretations but also not well - received by the general fanfiction - reading audience.
Well, Yukio acquires powers in the manga. These powers play a crucial role in shaping the plot and his character arc. They give him new abilities and challenges to face.
If Yukio was a girl, it could open up new storylines and themes. For example, how society perceives her or how her gender influences her goals and choices could be explored in the fanfic.