Hello, RachelK, and welcome to Textkit !
To the best of my knowledge, Koine Greek is somewhat simplified Attic Greek with some hebraisms in the case of the Bible. I have never been surprised so far by any Koine Greek grammatical form. Only the style can sometimes differ from the Attic usage.
You could maybe read of Lucian of Samosata, an author from the 2nd century BC edit : please read AD
that wrote very classical Attic Greek. Here is a sample :
It is a little dialog between two wind-gods : Zephyrus, the west wind, tells Notus, the south wind, that he saw Zeus kidnapping Europe. Enjoy !