The first thing I should say is that new sources - both material and textual - are constantly being discovered. The study of ancient Mesopotamia has a shortage of researchers rather than of sources, with vast amounts of evidence still entirely unexamined. More traditional areas of research like Greece and Rome also enjoy an ongoing expansion of the evidence base. Roman settlements and fortresses are excavated all over the former Empire and beyond; just last year, a papyrus dump in Egypt yielded fragments of a previously lost work by the Archaic Greek poet Sappho.

That said, it is true that most ancient historians do not work with substantial amounts of new material. Mostly, such finds are exclusively known to the finders or the people working on the project until the results are published. So yes, we are mostly working with a known set of sources, and literary historians like myself are travelling and especially well-worn road.

Nevertheless, it is always possible to create narratives and theories that make more sense of more of the evidence. This is particularly true in ancient history, where sources are often fragmentary, scattered across time, and representative of only a narrow range of interests and backgrounds. Better ways to understand these sources, to connect them to other forms of evidence, and to place them within larger stories about the ancient world, are constantly being devised.

It would be idealist and naive if I claimed that our ongoing work is gradually bringing us closer to "the truth". We will never know for certain what it was really like to live and work and feast and fight in the ancient world. The cynic would say that we are all hopelessly blinkered by our own biases and frames of reference, and we are merely producing the version of ancient history that is appropriate for our age, just like every generation did before us. However, there are some grounds on which it is fair to say that our view of the ancient world is progressing. Recent ancient history is more inclusive of traditionally silenced voices (slaves, women, non-citizens, 'barbarians'), more thorough in its methodology and theoretical underpinnings, and in some ways less ideologically motivated and therefore hopefully less distorting and selective than some of the Marxist or neoconservative narratives that have gone before.

Every single subfield of ancient history is progressing in this way, and our understanding of the Greek and Roman past is changing all the time. Almost everything you'll find in a basic textbook on ancient history is now outdated, but only a few specialists know this until someone writes a new synopsis that takes it all on board. For my own field of Greek warfare, I wrote a post a while ago that you might find interesting.