With Highschool DxD ending with Volume 25, light novel series author Ichiei Ishibumi and illustrator Miayama-Zero have already announced the impending sequel. Instead of releasing Highschool DxD Volume 26, the author will transition to the new Shin Highschool DxD light novel series, which publisher Fujimi Shobo has scheduled to release in the summer of 2018.
The Japanese title, Shin High School DxD (or Shin Highschool DxD), literally means the "genuine," "true," or "real High School DxD" when translated into English. All that is known about the story of Shin Highschool DxD is contained in the afterword written by Ichiei in Volume 25 (please see this link for a fan's summary of the book).
The High School DxD sequel could be likened to how Wolf And Parchment continued Spice And Wolf, although the relationship between the two series is probably closer to Naruto and Naruto Shippuden since the DxD sequel is apparently not focused on new main characters. Based on the afterword, the author is resetting the volume count largely because the publisher wanted a new series but Ichiei wanted to continue the story beyond the "final" story arc. Oddly enough, that means the first series is "ending" in the middle of a tournament story arc so that means Shin Highschool DxD could also have a high volume count.
Warning: Major spoilers ahead for anime-only fans or light novel readers who have not finished the book series.
The next book will continue the tale of the Azazel Cup (a world tournament that serves as a proxy war where all mythologies can compete), but Ichiei is creating a demarcation point between the series because the first series focused on one world or universe, whereas Shin Highschool DxD's story focuses on a conflict with another dimension. The story concerns the invasion of the UL (Underworld Lifeform), which is a race of creatures from another dimension. This concept was actually introduced back in Volume 7, when main protagonist Issei Hyoudou invented the Pailingual/Booblingual spell, which enabled communication with Chichigami unknown to the gods, devils, or dragons of the main dimension.
Fans believe that Shin High School DxD Volume 1 will adopt a more serious tone, but 18-year-old Issei is still striving to be the ultimate Harem King in any dimension. Issei became engaged to both Rias Gremory and Akeno Himejima in Volume 22 and proposed to Asia Argento, Irina Shidou and Xenovia Quarta in Volume 23. However, wedding bells have still not rung in a marriage, despite the fact that Issei has long talked about wanting to marry Rias. It could be argued that marriage is a foregone conclusion considering that High School DxD EX literally had Issei's children from the harem travel back in time from 30 years in the future to help defeat Loki.
The light novel series has inspired four seasons of anime. The latest anime adaptation is called High School DxD Hero, and was released during the spring of 2018. It's believed that High School DxD Season 5 will bring the Heroic Oppai Dragon story arc to a close. A Shin Highschool DxD anime adaptation will probably take many years to happen because the first story has about six anime seasons of source material left to cover.