Boruto: Naruto Next Generations manga storyline has been on fire as of late, dropping major revelations about the series storyline - as well as opening up new revelations about the larger Naruto saga and its Shippuden war. In the midst of learning the true nature of the Otsutsuki Clan and the threat they pose to Boruto and Konohagakure, Naruto fans have also been hyped about a subplot of the "Kara Arc" that's quickly growing in importance: the identity of Koji Kashin. The Kara organization's mysterious enforcer has fans believing he's actually Jiraiya reincarnated, which we be a huge (but not unwelcome) move for the Naruto franchise.
This is why it's good for Boruto to bring back Jiraiya:
Anime Test Run
First things first: we've already gotten a nice test run of how Jiraiya fits into a Boruto setting, courtesy of the anime. The Time Travel arc saw Boruto and Sasuke travel back to Konohagakure at the time between Naruto and Naruto: Shippuden. There they met and interacted with Jiraiya, and Boruto got to know why his father's mentor was so important. It was some of the Boruto anime's richest and most interesting character work, and it sets up a great juxtaposition with Boruto's current encounters with Kara and Koji Kashin.
Koji Kashin Isn't "Jiraiya"
Even if it's revealed that Koji Kashin is somehow Jiraiya returned from the dead, it doesn't mean that "Jiraiya" has necessarily returned from the dead. Boruto has made it very clear that Koji Kashin is the exact opposite of Jiraiya in terms of personality and temperament, cold, stoic, and ruthlessly strategic. Even if someone were to confront him with the revelation that he used to be Jiraiya, that wouldn't suddenly make Koji Kashin magically morph back into his former "Pervy Sage" self.
The real emotional pay dirt that fans seem to want out of this possible Koji Kashin / Jiraiya connection is to see how Jiraiya's resurrection will effect Naruto. The core of the original Naruto series was found in Naruto's relationship with Jiraiya, so there is so much emotion still packed into that bond. No matter how the revelation would play out, Naruto would be vulnerable in a way he's never been before - and totally mind-screwed if the "Koji" persona has somehow overtaken what used to be Jiraiya.
Boruto's Best Teacher
Having Koji Kashin / Jiraiya back in the picture would give Boruto a big opportunity to not only have the same teacher his dad did, but also a much deadlier and edgy version of that teacher. The scenes of Koji trying to teach Boruto and/or Kawaki how to fight, summon, use the Karma seal, or generally operate as ninjas would be as exciting as they are hilarious.
If it turns out that Koji Kashin is actually Jiraiya back from the dead, there's definitely one thing fans will want to see, besides his reunion with Naruto: a full Sannin reunion! The Jiraiya, Tsunade, and Orochimaru have never worked together since the Second Shinobi World War, but with a new Otsutsuki threat now hanging over Earth, and Koji standing at the forefront of stopping it, this would be a fitting time for the Sannin to make a (last?) stand. Plus: we already suspect that if Koji is Jiraiya, Orochimaru might have something to do with it...
Bottom line: having Koji Kashin revealed as Jiraiya would be a fantastic x-factor twist, which would quickly shake-up the character dynamics of Boruto. Even after the battle with Kara fades, Koji / Jiraiya would be a great dynamic character to have around - never quite a friend, or quite a foe, but someone pivotal and important in between. Seeing the other characters of Naruto / Boruto react to that new reality would provide great content for a long time.
Death Hurts Twice as Hard
Of course, Boruto doesn't have to keep a resurrected Jiraiya around for long - and arguably may not want to. Finding out that Koji Kashin is Jiraiya back from the dead will be a major blow to Naruto and co. - but it would be an even bigger blow to find Jiraiya, only to lose him again! Right now the Boruto manga has made things look pretty grim for Koji Kashin, who is battling Kara's leader Jigen (who is secretly Isshiki Otsutsuki) in a one-on-one duel, with no apparent support. Naruto has already begun to sense how much Koji is connected to Jiraiya's fighting style - it would be brutal for him to confirm his suspicions, just as Koji gets destroyed by Jigen.
Brave New World
Then again, even if Koji Kashin doesn't survive his battle with Jigen, there's no guarantee that whatever process was able to bring Jiraiya back as Koji Kashin can't be used all over again. As stated, there's suspicion already being dropped that Koji Kashin was designed (not born) as a weapon against Jiren - a living weapon that could be bashed on Jiraiya. Who's to say he's the only such weapon in stock? All along Boruto has been exploring what kind of modern forms of artificial life are being developed - Koji Kashin could be the prototype of a whole new form of an elite undead soldier.