According to HMV, the fifth volume of C³ Blu-ray will carry an unaired episode. It will be released on April 25th.

According to the official website, each of the six volumes of the Highschool DxD Blu-rays will include a short anime "Mousou Bakuyure Kaijyo Original Video (Fantastic Boob Jump Unlimited)". The first volume will be released on March 21st.

PV of Highschool DxD (Youtube)

According to Neowing, the second and the fourth volumes of the Mirai Nikki Blu-rays will carry unaired "Ura Mirai Nikki" anime (the short episodes of Mur Mur inserted at the end of the main episodes). The second volume will be released on January 27th. More unaired Ura Mirai Nikki will be released in the later volumes.

According to the official website, the first volume of Ben-To Blu-ray will carry two short anime "Shiraume Ume no Sadistic Gekijyo" and "Oshiroi Hana no Muscle Gekijyo". They will be released on December 21st.

PV of the extra short anime (the first two)

According to the official website, each of the six volumes of the Maken-Ki! Blu-rays will carry a 10 minute long short anime "Doki Doki! Maken-ki! Himitsu no Kunren". Secret trainings with the main characters will be animated.

PV of the extra anime (Youtube)

Sources: HMV, Highschool DxD official website, Neowing, Ben-To official website, Maken-ki! official website

News credit: Yumekichi11

C³ Special on MAL
Highschool DxD Specials on MAL
Mirai Nikki (TV): Ura Mirai Nikki on MAL
Ben-To Specials on MAL
Maken-Ki! Specials on MAL