Izuku replaces Issei, but instead of being perverted, he is an extremely shy/bullied child, simply because he doesn't have a quirk. This follows the story events of both dxd and mha.
Order of events:
Izuku gets killed and becomes a devil
Slime villain (he starts training the next day)
Learns about devils during stray devil event
Meets Ibarra (Asia replacement)
Freed first encounter
Ibarra's kidnapping
Ibarra's rescue
The entire riser incident
The entrance exam
USJ
Kiba goes rouge
The sports festival
Stain
The finals
The summer camp
Spend the rest of summer in hell (Loki and all that)
The overhaul event
The tournament between Rias and her cousin
*The rest are mha events combined with custom dxd events*
Izuku x harem