Sasuke often appears in different story arcs of the Boruto manga, depending on the current plot and mission. Sometimes he's on a solo mission, sometimes he's involved in events related to the village.
Naruto is likely to be popular too. Even though it's a Boruto x Sasuke fanfic, Naruto's relationship with both of them is so important. He could play a role in supporting them from the sidelines or even getting involved in a crucial part of the story.
In this kind of fanfiction, Itachi might use his powerful Sharingan abilities. He could perhaps create illusions to mislead Orochimaru and his minions, then quickly snatch Sasuke away. For example, he could make Orochimaru think that Sasuke is in a completely different location while he secretly approaches and rescues him.