AU:
Teyvat is an island that is unknown to the world and even the supernatural.
Aether or know as the traveler, who traveled world to world and got stuck in teyvat by the unknown god and his powers got seal and his sister is nowhere to be seen because that, he took an oath to his sister by traveling to the seven nations and also saving the seven nations and while that's happening. He and his sister got a small reunion and also finding that his sister the abyss princess who is the enemy of teyvat and also while that's happening a certain traveler don't know that he accidentally created a harem without him knowing because he was dense and that a numerous girls from the different nations has took a liking to him and also their a yandere. Time skit to the future, the traveler save seven nations and teyvat, and live with his harem whose basically confession him and lumine wanted to kill them because her brother is belong to her (oh sweet alabama) and our traveler is currently doing a mission an accident happen. While using the way point, the way point is manufacture (don't question it, it's logic)and teleport outside the teyvat and losing his memory (again logic) and marriage couple found him and take him to their house and our traveler woke up and the marriage couple question him what is his name and aether who basically has amnesia don't know his name and the marriage couple decide adopt him and name him... Issei Hyoudou.
And done bye-bye.