RE copyright: Copyright protects the specific words in a story, not an idea. If you took someone else's book and rewrote it all in your own words, changed the names of the places and characters, etc, your book might be totally unoriginal, but it would not violate the original author's copyright. There can be hazy cases like, for example, if you write a song that copies key phrases from someone else's song. I suppose if you wrote a book where you copied someone else's book and changed 90% of the text but kept 10%, that might be iffy. (I wouldn't try it.)
But if, say, you wrote a story about a young boy who discovers that he's a wizard and goes off to wizard college and fights evil wizards, the fact that your story sounds an awful lot like Harry Potter doesn't make it a copyright violation, not unless you copy whole sentences and paragraphs from Harry Potter.
Now all that is quite a different thing from saying that people might say your book is a rip-off and unoriginal because you just stole the whole story from Metamorphoses. You can write a story that steals IDEAS right and left and you won't violate anyone's copyright. But it might still be a lame and unoriginal story.
That can be very subjective. I don't think that every romance is stolen from Romeo & Juliet or that every scene where someone is murdered around water is stolen from Psycho or that every story that has multiple points of view is copied from Roshomon. But I've also seen plenty of books and movies that WERE obvious rip-offs of some hit. You have to ask yourself, What new idea or new twist am I adding here? If you're just taking someone else's story, changing the names of the characters, and shuffling some scenes around until you're sure you're safe on copyright, that's one thing. If you say, Hey, this other story was very interesting, but what would have happened if ... Or, but wouldn't it be interesting to write a story like that but where there's another character who ..., then you could have an original story that just happens to borrow some ideas.