I'll never stop, bring back One Piece DxD! And I have an idea: when they go to the meeting to see Seirogog (you know, Rias's cousin), Garp, Luffy's grandfather, appears as his trainer. I can even imagine their faces when they meet Luffy's grandfather, especially Garp's face when he realizes he's now a great-grandfather. I can even see him being all affectionate and spoiling Lulu, which would make Luffy feel strange.
I also imagine them getting scared seeing how strong Garp is and Koneko asking to be trained by Garp, I even imagine Garp meeting Luffy's Harem
I can even see possible ships, like Katakuri x Kuroka, Sanji x Irina, and Koneko asking to be one of Luffy's lovers.
I can also imagine when all the straw hats go to the dxd dimension