Choosing a really good research topic is crucially important. The topic itself determines the success of the project, in that if well-enough chosen, it will be something you can actually do, as well as being something that needs to be done. The trick is situating a topic between broad and narrow. A very specific topic can have broad implications, or simply be a narrow question that doesn't lead to anything else.
What have been my topics?
(1) The work of the most significant Spanish poet of the postwar period - Claudio Rodríguez- with an inadequate bibliography (at the time). Mine was the first book in English on Claudio Rodríguez. It still is the first book in English on him, in fact. It will always be that. With a single author project like this, you want to make sure it's not the 18th book on Virginia Woolf if it isn't super-original in approach.
(2) My second book was on linguistic self-consciousness, what we used to call metapoetry back in the day. It was a good topic because it was fashionable--but that also makes it a bad topic because it probably hasn't aged as well. A multi-author book, with some canonical authors. This is the book of mine owned by the most number of libraries. It had the first significant writing in English on Gamoneda, the greatest living Spanish poet.
(3) My third topic was masculinity and gay identity in Spanish poetry. I published a few articles on this topic but it never became a book. It was a good topic because there was all this great queer theory being done. It was a different sort of topic for me after the first two. The downside was that I never quite made it cohere as a book and have only five articles to show for it.
(4) My fourth research project of my career began with a negative response to the "poetry of experience" in Spain and tried to account for Spanish poetry between 1980-2000. It was good because it brought me some notoriety and fame in Spain, some friendships and some animosities. This is the work with which I've had the biggest impact. My only regret is that it takes attention away from other things I've done.
(5) My fifth topic was the presence of Lorca in US poetry. That was a really good topic, perfect for me personally. It allowed me to show off my chops in two poetic traditions. It was the gazillionth book on Lorca, but it was wholly original.
(6) My current project is What Lorca Knew. The drawback is that I haven't yet been able to define it in very simple terms. It's a good topic for me, but does anyone else have a reason to care? Is it too much a repetition of 2, 4, and 5? I'm going to take the attitude that the part will justify the whole. Even if it is never a book it should be 8 articles.
There are other aborted projects along the way. A book on Valente never took shape, though I've written five articles or book chapters on him.