We know that the True Longinus has God's dying will within it.
What if, this dying will has a purpose? And that purpose was why it chose Issei over Cao Cao in their fight?
Be interesting if the purpose was to find God's successor (with Michael being meant as a regent until said successor was found) and decided Issei would be the next God. Transforming him into one.
Be interesting as an idea for a fan fic if nothing else.