Author has written 9 stories for Sekirei, High School DxD/ハイスクールD×D, and Highschool of the Dead.
Ideas for others (Please PM me if you take any of them up I'd like to see how you go through with them)
Highschool DxD - OC replaces Issei, who has Divine Dividing and an OC Gear which holds Tiamat meaning Vali has Boosted Gear. Try and avoid the traditional with the OC Gear
D Gray man/ Highschool DxD - The second Exorcist project occurred at the same time as the Holy Sword Project. OC that would use Kanda's Mugen.
Eva/ Megaman Starforce - Omega-xis meet up with Shinji before he arrives at Tokyo-3 and together they have to face down the FMians and the Angels
Highschool DxD - The failed experiment to make the ultimate Yokai produced someone who could change between 10 different ones instead. [Ben 10 influenced idea]
Highschool DxD - OC who has no Sacred Gear or any special talents joins Rias' peerage as a Pawn along side Ise. Gets stronger through getting mythical/ special weapons and learning ancient magic.
Highschool DxD/ KHR - Tsuna has the Boosted Gear and Reborn shows up during his second year at Kuoh.
Highschool DxD/ KHR - The Vongola is a Clan of Extra Demons and Tsuna is the unaware heir until Reborn shows up on his doorstep. (Mixture of KHR and DxD plot)
Eva/ KHR - Shinji is the heir of the Vongola Family and Reborn shows up on his doorstep after the Seventh Angel.
Highschool DxD - Oc with an Sacred Gear that has an Angel sealed inside.
Harry Potter/ Highschool DxD - Harry Potter is the reincarnation of Yuuto Kiba and gains his abilities. Maybe start at some point before Goblet of Fire.