Like, if I wanted to write a book on Roman warfare, would there be like an overcrowding effect? Are would be historians encouraged to go into certain subjects because of overcrowding effect or is there still room everywhere?
Rome and Europe have been written about ad nauseum. There are still worthwhile subjects to be tackled in that region, oh yes, but the popular stuff gets regurgitated.
Want an area of study? Map out the Mississippian Civilizations. Flesh out the trading sphere for the Anasazi. Find an example of how the Inca used to socially engineer newly conquered societies in order to incorporate them into the empire. Define the ecological package of plants and animals that Spanish or English explorers if the new world would bring with them. Pen a pre-Columbian history of the Amazon societies, which are completely unlike the lightly populated tribes we know of today.
And then, take that stuff to the old world. You know, I can't think of any work on the Roman ecological package.
Just like everything else in life, there is a relationship between supply and demand. Topics such as "general Roman warfare" may seem to be over-crowded but it is a popular topic. You may choose instead to write specifically on the Siege of Groenlo, or even more specifically on the Siege of Oppenheim, but you won't have as large an audience.
I always found the Byzantine Empire was a lot less well documented than other kingdoms/empires, even though it was historically a very important military, religious, political and economic power.
You're gonna have to learn Ancient Greek to study it though, so you'd have to put quite a lot of efforts as it's not the easiest language to learn. But overall, in the western world, it really isn't the most popular subject, even in medieval history.
How about Thule/Dorset history?
Great question. I've been searching for topics that aren't part of a saturated market myself. My area is American history, the Civil War era specifically. It even seems like every possible micro history has been done by someone.
I think any topic has some potential so long as you have access to research. If you are an independent historian you better have deep pockets for travel and copies of research items, or be lucky enough to live near some untapped resource.