There are certainly a lot of writings worth considering for adding to the biblical canon (Just look at Aric’s cheater list). But my vote is for something a little more ancient like the Didache. It is a book that predates many that made the New Testament, with scholars placing it’s writing as early as 40-60 CE. It contains many descriptions of early church practices, around things like communion, baptism, and codes for ethical living. It is a fairly unique book in that it offers teachings from the early Christian community that seems unfamiliar with the teachings of Paul. It was a book up for consideration in the canon and ultimately was excluded. While I personally struggle against some of the teachings that reside in this short book I think it’s about time more Christians struggle with it. Here’s a translation you can read online.
I think we should just open the canon wide, maybe create a second category for midrash, journals, and debate, like the Talmud or the huge library of Buddhist sutras that aren’t Siddhartha’s teachings. There are a lot of pieces I could see fitting there like John Woolman’s Journal, or Scivias by Hildegard of Bingen, or MLK’s Letter From a Birmingham Jail, but if I had to pick only one (I know I totally just cheated) I would pick George Macdonald’s sermon Justice. You should read it right away. After all I just said it belongs in the canon. Is there a higher compliment I can pay it?
Functionally, the canon is open, and I think that we should just admit that and move on, relishing the endless beauty and meaning-making that surrounds us. We already make use of texts that are not in the canon more than those that are actually in there (hint: Calvin and Barth are not actually in the canon). As for what I would add, you already know what I’m going to say, because it is part of my canon right now: The Lord of the Rings needs to go in there, because it is like so much of the Bible – beautiful, deep, rooted in a person’s understanding of God, and imperfect.