"High School DxD" ran for 3 seasons from 2012 to 2015 and the fourth season is not yet confirmed. However, the possible storyline of the upcoming season is already released and Issei will most likely deal with another set of beautiful characters.
"High School DxD" is one of the most popular anime series that blends the supernatural with the harem theme. But even with its immense popularity, season 4 is not yet confirmed according to Dates Release and if the series follows its normal schedule of release, a new season should be available for viewing in 2016. But since there is no formal announcement to date, the fourth season could arrive next year.
But even though the continuation of the series is still unknown, the original LN or Light Novel series of the same name is still in print. If there's a Light Novel, then the studio has more enough sources to work with for another season.
If the fourth season is coming, the story line of Issei Hyodo will go through some serious twists. According to a forum on My Anime List, Issei will have to face Loki but help is coming from an unlikely source.
The upcoming season will have the biggest twist in the series because Loki will exploit his power to travel through time and change his past. In order to seal his victory, Loki also has to ensure that the events related to Issei do not happen at all.
Although Issei is unaware to this, he will have some help from the future. Specifically, Issei will be visited by his kids from the future. This is another set of harem but they are actually his daughters from his current companions. Each has its own powers to deal with Loki and they could defeat Loki with Issei's help.
There is no formal announcement for the fourth season of "High School DxD." If the series continues, the series will go through a series of twists especially for the main character. Issei will have his hands full as he tries to defeat Loki with the help of his time traveling family.
