The
canon has been established for centuries. Those letters and gospels that were written by apostles and prophets (whose authority was established either through their being in Jesus' inner circle or through miracles), tested by the community, and widely influential (and therefore widely distributed) were put in the canon, and those that didn't meet those requirements were not (even though they might be useful historical material.)
Every so often, somebody discovers a new writing from Bible times (or from hundreds of years after) that mentions Bible characters and they get it in their head that it'll be the next big thing. But, chances are, if the early church didn't take the time to preserve and copy and pass on the teaching, it's because the early church community did not consider it to be worth passing on. There are dozens to hundreds of ancient manuscripts for every passage in the New Testament, because those writings were copied and sent from church to church. It's doubtful there are any writings the church took as seriously as these that somehow got lost along the way.
There are tons and tons of other "gospels" and other writings by early church leaders that didn't make it in to the canon. There are also many writings by outsiders whose teachings were never accepted by the church at large, and that means they probably wouldn't be accepted by the church today either. Lots of people wrote about Jesus and about the early church, but very few of those writings deserve to be called "scripture". It's doubtful any newly-discovered writing will have any significant impact on the Christian community, though there are many anti-Christian "scholars" who will seize upon anything they can to discredit the Bible. Typically, such people greatly underestimate the wealth of information we have surrounding the Bible and the early church, as evidenced by the number of people who take the
Gospel of Thomas seriously. (If you're curious... read a few chapters of any of the Biblical gospels, and then read the gospel of Thomas. It should be abundantly clear why nobody in the church takes it seriously.)
My own prediction is that, unless they found a copy of "Q", nothing of great significance to Christianity will come of this. (Q is the name given to a writing many people hypothesize must have existed and been used as source material for the
synoptic gospels. It would be fascinating if it was found to actually exist.)
