It's a shame that there wasn't room for Hifumi. She's a great character, and although her talents shine through in her Confidant abilities, she would be even more compelling if she was explored more deeply as a Phantom Thief. Fans love to speculate about what Hifumi would've been like if she had played her original role. There's tons of fan art out there speculating her Metaverse look and her possible Personas, for instance. It's hard to say for sure what exactly Hifumi's abilities would've been like, but there's enough clues and details about her character that make the Phantom Thief Hifumi easy to imagine.
Aesthetically, it seems likely that Hifumi would have had a lot in common with Yusuke in the Metaverse. She seems to have a lot of interest in Japanese traditions, considering her mastery of Shogi and the knot she wears in her hair, similar to a knot for holding a certain kind of Shinto amulet. Hifumi frequently talks about imagining herself as a queen leading her army while playing Shogi. It stands to reason that, as a Phantom Thief, she would wear something that made her look like a queen. Perhaps she'd have an outfit inspired by the regal gowns of medieval Japanese empresses, but stylized and made practical for fighting and stealth.
Hifumi's weapon of choice in Persona 5 probably would've taken notes from Japanese history as well. While she couldn't just copy Yusuke's katana, there's lots of other weapons she might master. For instance, she could wield a pair of jitte; making for a graceful but strong melee attack somewhere between Joker's and Makoto's. As for a gun, the Shogi master would need something else that displays her skill and the power of precision. A sniper rifle seems like a perfect choice. Hifumi could have had a small clip, but a highly accurate and powerful gun that would stand out from the rest.
It's hard to say what Hifumi's Persona would be, but one feasible contender would be Kaguya Hime. This figure from Japanese folklore defies her suitors and bears a connection to the heavens that seems fitting for the Star Arcana. As for abilities, Hifumi seems like the kind of character that would focus on buffing allies and debuffing enemies as a utilitarian support. That could be supplemented with status affect abilities, the protective Tetrakarn and Makarakarn, and maybe a not insignificant amount of Bless damage.
It would've stood to reason if Hifumi's introduction came right around the time that Persona 5 added Makoto to the party. In a world where Persona 5 included Hifumi, there might have been a greater emphasis on Kosei High. While investigating unsavory events there with Yusuke's help, she might have become entangled in the Thieves' plans, but shown her tactical wit alongside her Persona.
Theoretically, it could have been up to the Phantom Thieves to stop a government official in the Ministry of Education from hushing up scandals all over Tokyo. Lots of Persona 5 fans wanted to change Principal Kobayakawa's heart, so maybe he would have come into play. A Palace transforming Kosei High or the offices of the Ministry of Education into a Japanese-style fortress would be pretty exciting to explore.
Hifumi could have turned out to be the most fervent of all the Phantom Thieves. Whenever she's playing Shogi with Joker, there's an intense side to her that comes out, eager to do battle and crush the enemy. Her intelligence and tact in battle would be equally matched with a steely regal presence. It's a shame that there wasn't room for Hifumi to join the Phantom Thieves. Admittedly, Makoto would have to change a lot to fit them both into the team, and it's hard to say what Persona 5 would look like on the whole if Hifumi made the cut.
Even though she's not on the team, Persona 5 is made better by Hifumi's presence. She'll always be loved as a strong character, a valuable Confidant, and a trustworthy ally throughout Joker's time as the leader of the Phantom Thieves.
Persona 5 and Persona 5 Royal are available now for PS4.