The story of Highschool DxD Season 5 is fairly predictable now that anime studio Passione has taken over the anime adaptation starting with Highschool DxD Hero (or High School DxD Hero). The light novel series by author Ichiei Ishibumi and illustrator Miayama-Zero sets down a solid path to follow and it seems very likely that Passione will toe the line.
Unfortunately, the third season, Highschool DxD BorN, became infamous for veering away from the canon source material near the ending of the season. Anime studio TNK not only squeezed three light novels into a single 12-episode season, which greatly rearranged the presentation of the story, they also changed the story completely for Episodes 9 through 12. Thankfully, Passione director Yoshifumi Sueda started Highschool DxD Hero with Episode 0, which provided a recap and retconned the last several BorN episodes out of existence.
What's more, when the author published Highschool DxD Volume 25, he announced via the afterword that the Highschool DxD Hero anime would be covering only Volume 9 and 10. The fourth anime now resumes the anime's pacing of the first two seasons, which also covered two book volumes per season.
That also means it's easy to predict what comes with Highschool DxD Season 5. The light novel series ended with Volume 25 in 2018, so the fifth season will very likely cover Volumes 11 and 12 and bring the Heroic Oppai Dragon story arc to a close.
High School DxD Season 5 Release DateAs of this article's publishing, animation company Passione has not announced anything official about the High School DxD Season 5 release date, although this article will be updated as soon as the anime sequel's premiere is confirmed. However, there can be reasonable speculation about when, or if, the Highschool DxD Season 5 premiere date may occur.
Oftentimes, a popular series is forced to wait until the manga or the light novel series provides enough source material for another season. That's not the case with the Highschool DxD light novels. Author Ichiei was already up to Volume 12 way back in 2012, which is when the first anime season aired. There are already enough books for six more anime seasons on top of the fifth season.
Assuming that the series' popularity leads to long-term financial success, even then anime projects take years to develop. Most anime studios are booked out for projects years in advance, and studio Passione is no different. In 2018 alone, studio Passione released the Citrus anime during the winter and both Highschool DxD Hero and slice-of-life comedy Hinako Note released in the spring. Their next project that's been announced publicly is Z/X Code Reunion, which is slated for an October 2019 release date. There's also Project [B.B], but the premiere is yet to be announced. As such, the Highschool DxD Season 5 anime probably should not be expected until 2019 or 2020 at the very earliest.