Someone I met recently asked my why I was writing two more books on Lorca. I said that I had a lot more to say about him. Even without doing more actual research, per se, I have a lot of ideas. When you have been reading, teaching, writing about a poet, for three decades you tend to have more to say, not less. I only decided to become a full-fledged lorquista in 2006. Before that, I taught his works, and did a little research in the 90s that didn't turn into very much, a few conference papers, some rejected articles. I had always taught Lorca's theater in introductory classes, and his poetry in a variety of settings.
I actually think I could write a fourth book on Lorca. I don't know what that book is yet, but I am sure it will emerge into my view once I write these two more.
You never say everything you know, anyway. What you publish should be the tip of the iceberg, supported by a deeper understanding that isn't going to be fully articulated. Then your work will have more depth to it. What you write will be a selection of your best ideas, presented in the best possible form.