The first idea I got while reading the first chapter of a High School DxD x Final Fantasy IX story, Black Mage of Kuoh By: Quatermass ( https://www.fanfiction.net/s/11897451/1/Black-Mage-of-Kuoh ) & listening to this:
It's where Issei is the reincarnation of the original Godzilla/Kiryu, whose soul came from another dimension. He's triggered by hearing the roar of Godzilla from a movie (yes, in this story, the Godzilla movies exist in High School DxD world). Also in this story, Issei isn't a pervert. Also, he's discovered his Sacred Gear on his own.
2. Second Idea, I got from listening to this:
Here, Issei is the adopted human son of Mothra. How this comes to be? It starts out in 2002 or 2003 as a very young Issei (probably around 3 years old) is on a cruise with his parents in the south of Japan. A storm strikes & sinks the boat. He's washed away & his parents killed. He eventually washes up on Infant Island. He's found the natives. They bring him to Mothra to decide wha