As I have written in the last chapter that Haruto learns Nen, then there is no reason to send him to Hunter X Hunter world as he can go to One Piece World directly. I was also thinking of going to the following worlds-
1. High School DxD - strong opponent like Great Red and Ophis.
2. Seven Deadly Sins - Lots of strong fighters
3. One Punch Man - Saitama, who has basically God-Level Power.
Please share your thoughts and tell me in the comments if this is fine or you would like to add other anime worlds.