One thing I did that worked well for me was to focus all my research papers on one subject all through university. In my case it was the study of Japanese prisoners of war held by the Americas. Every essay fit around that theme no matter what subject. I talked it over with the professors and offered to show them previous essays so they knew it was an original tack. So, Colonial US (nevermind) history covered POWs in that period.

Someday, I'll finish the book.

The idea is that you never have to go too far to develop an essay idea and the whole idea of PH.D is to become an expert in one subject so start early.