First of all, let me reassure you that you are not alone. Most of the academic historians here can attest to wishing we were experts in a list of fields. I'm already a medievalist who cheats on her main research with the Reformation, which is to say I have devoted the time and effort to command both primary and secondary scholarship on specific topics within both medieval and early Reformation time frames, in addition to methodologies of working with the different sources and dealing with language issues (medieval German is not early modern German isn't quite modern German). I dabble in late 19th/early 20th century gender history and early modern African history as well, and I wish I had the competence to do original research there, but I don't. At least not yet. Et cetera, et cetera.

And based on all this, I will say: it is not easy to get to the point of being able to produce original, publishable research across multiple fields. You have to know the published scholarship (what's been said) on your particular topic, but you also need a command of the general historical context of the setting, which is harder than I generally want it to be. You have to know the primary sources, the languages, the handwriting, the iconography, and so forth.

A common type of pop history book--almost never academic-quality, though--is the long-term "microhistory." A book like The Dirt on Clean or Salt will focus on a single topic 'throughout history.' The problem with these tends to be what I mentioned above--the lack of understanding the specific context. I often find it hard to read the medieval sections of those books, because the authors just don't know how to handle medieval sources.

So, there is a subfield called world history. But note that this is a dedicated type of history with its own focuses and methodologies. That is, world history is not Oceania/Pacific history plus African history plus European history. It focuses on global interconnections, comparative history that transcends traditional disciplinary lines, and generally not across broad swathes of time. (Indeed, rethinking periodization lines within an individual time frame is one of the coolest results of world history research!)

The other thing is, "I love everything" generally just means you need more time to figure out what you really love. If you want to do a master's degree or even a PhD, you will need to focus on a specific topic for a significant amount of time (master's thesis, doctoral dissertation...I believe the terms are switched in UK-based systems). Even if it's a 'world history' field, you'll be working on a specific topic.

To get to that point--takes time and exposure to the material! I didn't know what I wanted to do when I started grad school, either. :) I loved six or seven different fields. The Middle Ages picked me, during the second semester of my MA. That's one of the best things about doing a (separate) master's degree first. It gives you time to mature as a scholar, but it also gives you time to figure out what you're actually interested in.

You might go on to a long and amazing career as an academic historian who starts off in one field and finishes in another. There are definitely scholars who do that! But first they figure out one place to start in.

Graduate historical education in academia discourages generalists for a good reason--it's really, really, really hard to develop the competences needed to conduct original historical research that will stand up under scrutiny. That doesn't mean you can't do it ever, but you probably can't do it within the time frame allotted for a master's+PhD.

/r/sunagainstgold outlines the problem with trying to be a true "universalist" quite well. Maybe you pull it off after 50 years of trying (see, e.g., the sweeping works of people like Eric Hobsbawm). But you can't start that way.

But I would point out, there is another way to go about this. You are listing your interests in terms of locations and periods, which is typical. There are thematic strains of history, though, that branch over locations and periods quite willy-nilly. The history of science, for example, does not constrain itself by period and location, although of course whatever dissertation one writes must be constrained somehow (for the sake of time/length). But a graduate education in history of science spans huge time periods. And indeed, allows me to teach a class on "the history of science and technology," which spans from paleolithic times to the present, looking at the entire world at once. Of course I cannot call myself an expert on all of these periods and places (the languages alone are enough to avoid that), but I'm interested in them not so much for their fine details, but how they fit into the theme. So I can say some things about the organization of knowledge and labor in the Ancient Near East, even if I would not by a million years consider myself an expert on the deeper subject of the Ancient Near East.

Other forms of thematic history include Cultural History, History of Technology, Environmental History (relationship of people and the built and natural environments), Economic History, History of Gender/Sexuality, Global/Transnational History, and no doubt many others that are not coming to mind.

I would add to the previous answers that there's also a lot of work being done in comparative history, though usually that involves co-authoring. A number of historians I know are currently working on/have published comparative work on topics like empire, economy, etc (eg Walter Scheidel). The thing is, you usually have to establish a pretty deep expertise in one area (and probably get tenure) before you can start doing work like this and get taken seriously.

So just because you have a focus, doesn't mean you have to give up other interests. Academia is not the ageless institution it looks like from the outside, interests change, and you're much more likely to succeed if you come in with a lot of interests than if you're stubbornly attached to one thing.