One of the things that the ancient Old Testament peoples had with the New Testament saints is that both believed in an open canon of scripture. In fact, there is nothing in the Bible that closes the canon of scripture. Yes, John warns adding to the prophecies written in his book (Revelation 22:18), but it was just that -- his book, and he was warning not to add to the prophecies in his particular book. Mose wrote similarly when he wrote Deuteronomy 4:2: "Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the Lord your God which I command you." Yet many prophets added to those words and we accept them as binding on us today.
During the battle of Armageddon, God will raise up two prophets who will use divine power to thwart the Beast and his armies. John writes in Chapter 11: " And I will give power unto my two witnesses, and they shall prophesy a thousand two hundred and threescore days, clothed in sackcloth. These are the two olive trees, and the two candlesticks standing before the God of the earth. And if any man will hurt them, fire proceedeth out of their mouth, and devoureth their enemies: and if any man will hurt them, he must in this manner be killed. These have power to shut heaven, that it rain not in the days of their prophecy: and have power over waters to turn them to blood, and to smite the earth with all plagues, as often as they will."
While they are engaged in their earthly missions, do you not think they will write concerning their ministries just as the ancient prophets did? And will their writings not be scripture, just as the writings of the early prophets became scripture? These two prophets will be killed by the Beast, but three and a half days later they will be miraculously resurrected. Certainly, like the prophets of old, these two prophets will write, and it's entirely likely that we will have their words during the Millennium. During the first century, Paul and other apostles frequently quoted from the Testaments of the Twelve Patriarchs, yet we do not have that writing in our canon. Why? Because many scholars thought there were too many Christian interpolations. But with the discovery of new documents in the Dead Sea Scrolls and Nag Hammadi writings (and others), there are a number of ancient writings that pre-date Christianity, yet sound remarkably Christian. We have the Epistle of Barnabus, which, among other things, strongly condemns abortion. Again, why was it left out? The first century Christians certainly considered it a part of their canon.
The Lord can add to the canon of scripture whenever He wishes. Protestant scholar Lee M. McDonald writes: “On what biblical or historical grounds has the inspiration of God been limited to the written documents that the church now calls its Bible? … If the Spirit inspired only the written documents of the first century, does that mean that the same Spirit does not speak today in the church about matters that are of significant concern?” (The Formation of the Christian Biblical Canon, rev. ed. (1995), 255–56)