(Hoping the CGI doesn’t bite me: )
You haven’t been watching the RCC. Even when the “understanding” changes, the original view is only seen in a new light. Nothing officially ever changes.
The Protestants are in the same boat. Luther went out and re-evaluated what should be considered Scripture and the various Protestant denominations have been fiercely holding to his thoughts for 450+ years.
At this point, the only way to get new Scripture would be for some group to break away from the RCC, the Protestants, and the Orthodox, completely. (One view would be that the LDS did just that.)
Even Luther only re-examined existing Scripture with an eye to removing the excess, not with the intent of adding new things. All of the big three Christian traditions–Orthodox, Catholic, Protestant–hold to a long-stated belief that NT Scripture must be Apostolic in character (written by someone of the first generation of Christians). Current scholars can use various literary analyses to decide that the early church made mistakes (based on that criteria) in their selection. (E.g., there are a substantial number of scholars who do not believe that the letters to Timothy or Titus were written by Paul.) However, once the canon has been established, there is almost no way to get anything new added to it.
The various letters of Clement, the Shepherd of Hermas, and other letters have already been separated from Scripture and categorized among the Patristics (the writings of the Church Fathers). It is not in the nature of any existing Christian group to change that.
To be newly added to Scripture, a document or text would have to be identifiable as coming from Jesus or one of his followers. I can think of no way that that could be accomplished (i.e., proved). The only other way would be for a group to separate themselves completely from one of the existing traditions in the way that the LDS looks to its Scriptures that are not in the Orthodox/Catholic/Protestant tradition.
Is it possible that the RCC or the Maronites or the Missouri Synod of Lutherans could consider another ancient text as Scripture? Sure. But it is rather less likely than that John Paul II will abdicate his post and join the Nation of Islam. I wouldn’t look for it to happen, soon. Anything might happen; some things could only happen in an alternative universe.
Tom~