There are two basically "tried and true" approaches to this. The first we might call "find a gap in the literature." That is, find a topic that, surprisingly, does not have a major monograph work about it already. This is what I did, essentially: it became clear to me, as I worked on topics related to nuclear history, that though of course everyone knew that secrecy played a big role in this history, there was in fact no singular study of nuclear secrecy. So that is what I did my thesis on. Sometimes one lucks out and such a topic just presents itself to one (and one is interested in it).

The risk for this approach is that, in many cases, as one of my advisors always warned, there are "gaps" for a reason: not every topic is worth working on. One could go endlessly "micro" in one's search for a "gap" ("ah, no one has written a monograph on Victorian clothespins!"). Thesis advisors are there, in part, to help one calibrate one's search for novelty — sometimes things that are not at all obviously interesting to non-historians (e.g., the history of Victorian pigeon breeding) can have interest to academic historians, in part because they end up touching on more than one might expect (Victorian pirogen breeding, as an example, was of great influence to Charles Darwin's thinking about natural selection, as well as to his cousin, Sir Francis Galton, on eugenics — so one can imagine there would be an historical account there that would tie into broader themes).

The second approach is one we might call "apply a new framework." Historians and philosophers and other smart academics are endlessly coming up with new intellectual frameworks. Some of these are pretty well-known throughout the academy ("Foucauldian analysis of power") and some are relatively new and emerging ("transnational history," which was all the rage a couple years ago). This approach to writing a thesis says, in essence, "I am going to look at a topic that has been well-covered by other historians" (like the Roman empire, for example), "but I'm going to be looking at it through the lens of X", where X is one of these philosophical/intellectual frameworks.

The danger of this approach is that you're just applying the latest fad to something already well-known, and not adding a whole lot to either theory or historical content. But that does not have to be the case, and new frameworks can provoke the historian to look towards new types of evidence, new types of arguments, and see different things.

Of course, one can hybridize these kinds of approaches, too: I'm looking at a gap, through a framework. This is not uncommon.

Through the course of research, one often does find "new evidence," though this is less about finding some new source that literally nobody has ever seen (that happens sometimes, but is very rare), and more about finding some source that has not been much appreciated by other historians. You probably won't be the first pair of eyes to look at a document in an archive (after all, someone looked at it at least once, to put it there), but you might be the first pair of eyes who says, "oh my, this is exactly what I need to make my novel historical argument!"

The above approaches are not the only possibilities, just the most straightforward and common to new historians. They are, in their own ways, somewhat uninspiring. (The reader will note that I put my own work into one of these categories, for whatever that is worth!) That does not mean they cannot and do not produce important, useful work (so I like to believe, anyway) — it just means that they are common approaches for novices. Occasionally people come up with much more inspired approaches; sometimes those pan out, sometimes they do not. Such is the nature of scholarship.

As one does more and more research (well past the PhD) one learns more and more both about one's topic of speciality but also broader ways of thinking about history. So the character of a historian's work can change over the course of their career, often for the better (this is sometimes contrasted with fields of science where the best work is typically done by the young). One can often spot a PhD thesis by its falling into one of the two categories above — these are reasonably good ways for someone who is still fairly novice to go about coming up with a novel contribution.

This particular way of understanding a history PhD isn't at all uncommon--but it's also not necessarily accurate.

For one thing, history is almost unimaginably vast. As I like to put it facetiously sometimes, "it's the study of the past, and to date, everything that has happened has happened in the past." There are tons of sources that no one has ever evaluated in a scholarly way, and we aren't likely to run out of them anytime soon. This is particularly true for people who study themes outside of the political history of Europe and its settler colonies, but even there you'll find material that people haven't gone through seriously. In my period, Robert Harley, who was the most important member of the British government from 1710-1714, left a massive corpus of papers that remain still poorly organized and lightly touched in the British Library. So while it is true that often we rely upon new sources for new insights, those new sources aren't as hard to find as you might imagine. This is especially true for people whose work relies on oral histories, since oral history has been frowned upon for so long in the academy that a lot of work in it will be new almost by definition.

But what of, say, ancient Rome? We aren't getting oral histories, to be sure, and new sources aren't emerging at breakneck pace. Well, the thing is that although you say "set in stone" in reference to history, history isn't set in stone at all. Sometimes the sources are unclear as to what the basic facts actually are, and a dissertation can be founded on reinterpretation at that level. But even when we have agreement on what the basic facts are, what they mean, and what they suggest, is frequently in contention. So there's room again to found a dissertation on a new understanding of what the facts mean when assembled. My dissertation is located there--we know just about everything that we're going to know about what happened politically in Jamaica and New York between 1675 and 1730, but I have a different analytical emphasis in my work than that you'd see in a lot of the scholarly work on the subject. Relatedly, there's the inevitable fact that history is always written through the lens of the scholar's own worldview and context. As those things change, the relevant weights of certain facts and patterns change. It doesn't make older history wrong or bad--though often it is--but it does mean that reinterpretation without new sources is still perfectly valid.

There's also the question of choices about combinations of facts and sources. I have a friend who's doing work on questions about the racialization of American policing that have all been studied in local contexts, but not in a comparative context, which is one way to create novelty. And there's also the choice to pay attention to sources that are known but have never been privileged--which often relates to what I mentioned above about changing worldviews.

I'm sure I've left out more methods by which one could create a worthwhile addition to historical scholarship, but I have a kid to put to bed. So I hope I've managed to show that while breaking new ground with new sources is certainly a viable way to make an addition to the body of historical scholarship, it is by no means the only way.

Well as someone currently working on their Masters before proceeding to the PhD level, I cannot agree with u/restricteddata's comment enough: those are the two main ways of writing a thesis or a dissertation. However, history is always expanding as a field because there's always new things being found. In my own personal research - into American Civil War soldiers and the experience of battle - I've been working a lot with archives that were surprised to find some of the things they had squirreled away in file boxes.

History is constantly being rewritten - I would add a third type of dissertation/thesis to what restricteddata listed, which is the research that challenges older research. In my own research experience, for example, the "Common Soldier" of the Civil War was examined in three well-regarded monographs - Reid Mitchell's Civil War Soldiers and Bell I. Wiley's The Life of Billy Yank and The Life of Johnny Reb. Mitchell was published in the 1980's, and is profoundly a post-Vietnam work of the Historical canon in what he chooses to focus on. Wiley's books were originally published in the 1950's as a Cold War history. So even if - impossibly - everything had already been written about, there's - generationally - a turn-over among historians who will challenge the views of the predominant schools of thought before them, reinterpreting and reanalyzing previous scholarship based on newly discovered or previously neglected evidence - and so separate from his second category.

The leaps and bounds that have been made in research tools (not in the least the personal computer) plus the work of archives around the world to digitize their collection is allowing for broader access to records than ever before - for my undergraduate thesis, I was able to access an after-action report for the Battle of Gettysburg that had been lost for over a century before a historian found it in a State Archives and helped get their collection digitized. Now everyone can access that record. This means that criticisms of Mitchell and Wiley - that they were hindered by the availability of records - may soon well be a thing of the past given the ongoing trend of the democratization of primary source materials.