New papyri are still being found, for example in the Fayyoum region of Egypt, so there is a good chance that there will be unknown literary fragments as well - it does happen from time to time, but of course most texts that are found are not literary in nature. In addition, there are still unpublished papyri in collections all over the world, and some of these contain unknown literary works as well.

But of course, papyri are essentially just 'scraps of paper' and in most cases the new discoveries are very fragmentary, so it is sometimes difficult to determine whether or not a new find belongs to a known literary work or is a newly discovered composition.

Serious question, how would you know? Is there some sort of index or by using past discovers as a basis?