How niche are you willing to get? I'm aiming for a flair of "Water in the Middle Ages", and I can currently claim a basic working knowledge of Medieval aqueducts.

Of course, the more esoteric your niche, the more difficult it can be to study. Especially since I'm entirely self-studying and don't have advantages of access, and geography works against me, in that I'm pretty far from any primary sources that cover my area. There is without a doubt someone (and more likely more than a few someones) who don't have any previous work that relates directly to their intended niche, and have to come at it from the side and do their own work that will then establish their niche for the future.