[The male protagonist transmigrated to another world or played a virtual game and couldn't return to reality, and he had a relationship with a cute girl or princess.]

Answer 1

There were many classic works that could be used as reference for this type of animation:
The story is about the fierce battle between humans and giants in order to protect their own city walls. Alan was a young giant hunter who accidentally traveled to another world and met Nausica, the female giant in the game.
2 Sword God Domain: The story is about a group of players who adventure in a virtual reality game and make friends with various characters. The male lead, Kirito, was a gaming expert who accidentally transmigrated to another virtual reality game and met the female lead, Yukino, in the game and started an adventure.
[3]<< Index of Forbidden Magic Books >>: The story is about a mysterious library that contains many books related to superpowers. The male protagonist, Tama Uejo, was a high school student who accidentally transmigrated into the world of the book and made friends with various characters to start a battle about superpowers.
4 " About Me Reincarnating into a Slime ": The story is about an ordinary office worker who transmigrated to another world and became a slime. He met the cute girl who reincarnated into the same world as the heroine.