The following is an edited version of my answer to the question, "Is the New Testament canon closed?" that turned out a bit differently than I expected: with a resounding "yes!".
Others on this forum and the text have said that Jesus is God's final revelation to us. In a sense I affirm this. Jesus is the true Word of God (Jhn 1:1) and the way, the truth, and the life that He wants us to know in Him (Jhn 14:6). But if Jesus were God's final revelation to us, why are there books in the New Testament after the gospels? Why is there a New Testament at all if the final revelation came in the form of a person?
The way I have come to understand it is that the New Testament both continues and interprets the revelation of Jesus. The Word that God spoke to our hearts through the person of Jesus when He was on earth, He continues to speak through the written Word (as well as the body of Christ, the church) after His ascension. Additionally, the rest of the New Testament is precious because it contains many insights into the ramifications of Christ's life, death, and resurrection that only became clear after the fact. So Jesus' status as the final, fullest revelation of God really has little to do with the question of whether other books could be in the New Testament canon.
Here is where I see some inconsistency: the text says that if Paul's letter to the Laodiceans were uncovered today, "to gain canonicity it would have to agree with all the other books of the canon and be accepted by the entire Christian church, which at this point seems unlikely." Yet just before Wegner says, "It is important to remember that the Christian church did not canonize any book. Canonization was determined by God. But the early church needed to know how to recognize canonicity."In other words, we are confusing two separate issues into one: the actual, God-given authority of books and the church's recognition of the books as canon.
In the first sense, I can say with certainty that all the canon books of the New Testament have already been written, because all the apostles are long-dead. In this sense the canon is closed. The second sense is intriguing: could a previously unknown letter from Paul be discovered and make it into the canon? To answer this, I want to clarify what we mean by canon. It doesn't simply mean "true writings about Jesus by the apostles", for in this sense it is quite arbitrary to draw the line at the apostles when plenty of later church thinkers have written true things about Jesus. (They hadn't seen Jesus in person, but neither had Paul) Rather, a crucial aspect of canon writings is that they have been formative for the liturgical, doctrinal, and practical development of the church from the beginning. The canon books of scripture do not merely inform about God, but God actively exercises His authority through them, which manifests in visible ways in the church (see Hbr. 4:12). N.T. Wright describes the authority of God through scripture as "his sovereign power accomplishing this renewal of all creation."
In light of this, I rule out the possibility that an unknown letter by Paul could be discovered and recognized as scripture, simply because such a letter would not have been unknown in the first century, at least to its recipients, and yet it was not circulated among the churches, adopted into church tradition, or quoted by church fathers like others of Paul's letters. Whatever "divine spark" Paul's letters to the Romans, Corinthians, and others had that led to their being included in the canon, his letter to the Laodiceans apparently lacked. We must not think of this letter's being lost to us today as an unfortunate canonical accident, but simply the early church recognizing what it did not have the same value as other letters and moving on from it once it had served its purpose. The canon that was good enough for the first-century church is good enough for us today.
So while the New Testament canon is closed both senses, it is important to clarify what this means. The Bible, whatever its contents, was never supposed to be the only writing about God, though it has played a unique role in the church from the beginning as the bedrock foundation for all writing, thinking, and believing about God. But the point of a foundation is to be built upon (cf. 1 Cor 3:10-17). That the later church fathers reference the writings of the apostles rather than trying to continue them speaks convincingly for drawing the canon line at the apostles, but certainly does not necessarily denigrate later writings of the church or even prevent them from having some lesser authority of their own. Saying that the Bible contains all the words we need to know God and be saved is a bit like saying that a small assortment of vitamin pills contains all the nutrients you need to survive. So let us trust in God's stewardship of the scriptures for our sake, while living our lives from the truth that they reveal.