Are there still gaps in what we know about widely researched topics in this sub like WW2, the Roman Empire, Medieval Europe etc? Is there such a thing as having a specific historical era being so oversaturated and discussed to the point of repetitiveness?

While there are some topics that have been discussed far more than any others like the American Civil war or the Nazis. A good historian researching any topic will be looking to find a new angle or perspective on the topic, either using new sources or a new theme, and most importantly trying to have something new to say that previous historians haven't, either by augmenting or rejecting the arguments of those previous historians