I love how you put: "God accepts anyone who comes to Him." This is the Gospel.

I don't know if you mean unique in history or unique in religious writings. There was an era in Egyptian history when the Hyksos (semitic peoples) and not Egyptians ruled on the Egyptian throne. I believe that Cleopatra was the daughter of someone who revolted against Egypt. There may be more.

But as far as religious writings, Christianity is really the only historical religion, so it would be unique in that sense that you're asking. Hinduism is a philosophical religion, as are Buddhism and Confucianism (though I wouldn't technically call either of those a religion). Islam includes some historical references, but the Qur'an is mostly a book of philosophical and theological snippets. That would make the Bible unique in putting one of their greatest kings as related to the "enemy."

You're right that Moab is an important part of the plot line. At least part of what's behind it (besides that it's historical) is that God accepts anyone who comes to Him regardless of their ethnicity, religious background, or family line.

Was the Bible unique in admitting that one of the greatest kings in a nations history (David) was related to the "enemy"

David is clearly said to be part moabite. Although it's a minor amount the book of Ruth has a very positive depiction towards a half moabite to the point where they show how she is a direct ancestor to David.i realize this subreddit probably won't give me the most objective answers but was this common in ancient history telling?