Yeah, the limited edition of volume 15 is already out, and 1/5 of the book was the main story which takes place after volume 14, and the remaining 4/5 are short stories. The book was structured in a way where the short story was more of a flashback that characters had during the story.
During the main story, some of the major things revealed was:
-Dulio and Slash Dog visits Hyoudou residence to support them. They were also told from Michael and Azazel respectively to become Issei’s sparring partner.
-Slash Dog’s Sacred Gear(Longinus) has a form of a dog. He was also the main character of Ishibumi’s second novel “Slash Dog”. He was the only other human besides Cao Cao strong enough to make Vali use Juggernaut Drive. He has the same physique as Issei, and appears to be one or two years older than Issei. His name is Ikuse Tobio. The dog(his Longinus) is called Jin.
-Dulio. A carefree guy who is really friendly. He decided to call Issei, “Ise”, just like the other characters close to him. He tellss Issei to speak to him casually and to call him Dulio. He appears to be three or four years older than Issei.
-Issei was suggested by Kuroka to consider taking Le Fay as his candidate as his magician. She can use all sorts of magic, and has a contract with Fenrir and Gogmagog. Ravel was against it since she belongs to the Vali-team, a group of terrorist. Though she only followed them because she was concerned about her brother.
-When the Gremory group was fighting Grendal in volume 14, the Vali team was fighting the other Evil Dragon, Aži Dahāka.The battle was halted because the two Evil Dragons started fighting each other.
-The new boss of Khaos Brigade is someone that Vali and Azazel knows. Vali spoked in a manner with disgust when he talked about him. Though the name wasn’t revealed.
-Rias and Kiba waits to talk to the head of house of Tepes. At the end of the volume, it was revealed that Valerie Tepes is the head of the House of Tepes, and the leader of Tepes faction.
-For the shorty story, there was six in total.
A short story about Akeno’s past and her meeting with Rias.
A short story about Gremory girls and Sitri girls auditioning as a magical girl together with Leviathan.
A short story about Vali team’s quest to help Albion because of the Oppai-Dragon things.
A short story about Issei, Akeno, and Gasper going to the Fallen Angels institution.
A short story about the Church Trio going to Akihabara.
And the short story about Kiba when he became Rias’s servant and when he met his master.
PS. Once the scans are up, I will start the translation.
EDIT: I also uploaded the first 2 chapters of “Koakuma Thiiri to Kyuuseishu!?”. I’m just going to do this series to have some fun in it. It will be sporadic translation.