"High School DxD" is a supernatural manga with a harem theme that ran for three seasons. Because of the unique story and impressive characters, fans and even casual anime watchers are waiting for a follow-up. Season three ran for 12 episodes and originally aired from April to June 2015 according to My Anime List.
Fortunately, there is no news that the series is canceled and according to release-date.info, the "manga is still ongoing, we may assume that the anime series will be renewed soon." Based on fan reactions on each episode online, the studio should be encouraged that it is gaining in popularity in every season.
With regards to the story, the latest rumors point to a very interesting story. The premise of the upcoming season is believed to be based on the one-shot manga "High School DxD EX" which offers a very interesting storyline especially for the fans of the hit anime series.
According to the Reddit Forum about the one-shot manga, "High School DxD EX" is a story about time travelers who will try to correct their future by going back to the past. These are not just your regular time travelers since they are revealed to be the daughters of Issei.
Apparently, Issei has been successful in creating a harem but the future looks bleak and his daughters have to time travel to correct things.
The possible plot for the fourth season is further supported by forum users at My Anime List. The forum's main discussion is on Issei's situation and the events that happened in the third season having a powerful impact on the upcoming season.
Loki would be working with various villains coming from a different dimension. Issei, his daughters and the rest of the harem would have to fight them in order to save not only the present but also their future.
