In a fanfiction, it could be a romantic plot device. Ino has had a secret crush on Shisui for a long time. One day, she finally gathers the courage to be so bold as to straddle him. This act could be the start of a relationship. Shisui is initially shocked but then realizes his own feelings for Ino. And then the story could follow their journey as a couple, facing challenges together within the Naruto universe.
One possible plot could be that Ino is in a situation where she needs to quickly gain Shisui's attention or stop him from doing something. Straddling him might be a very bold move on her part in the heat of the moment. Maybe they are in the middle of a battle or a dangerous situation where she uses this tactic as a last - ditch effort to communicate or make him listen.
Well, perhaps it's a story of a misunderstanding. Ino might think Shisui is involved in something bad (even though he isn't), and she straddles him to interrogate him. As she does this, she realizes she's made a huge mistake, and from there, a story of redemption and friendship can develop as she tries to make it up to him.