I've been running this site for more than three years now, about the same amount of time I spent on my PhD research.
When I first started, my idea was to write a book, "The Three Month Thesis" to help students write faster and with less stress. I'd written my thesis very quickly, and not only enjoyed the process but also wrote a high quality thesis the examiners were very happy with.
I actually wrote the book, but never released it because I wasn't happy with it. I didn't have enough experience. I hadn't worked with other students and tested the ideas. And I had made the flawed assumption that because I had done it, I knew how to teach others to do it.
So I shelved the idea and did other things. I took on coaching clients as my main focus, learning, making mistakes and developing ideas as I went. As a result, my perspective is very different now compared to three years ago (I no longer like the name "three month thesis", as just one example of many).
Whatever your project, your perspective should change over time as you gain knowledge and experience. This is a good thing, because it means you have learned something, but it also means you will have made decisions from a less-experienced standpoint which affect what you are doing (or not doing) now.
Sometimes, an idea doesn't work out not because it is a bad idea, but because it's the wrong time, whether because of external factors or just because of experience. And sometimes these old ideas can become viable when you revisit them with a new perspective.
Are there any old ideas you chose not to pursue in your research? If so, are the reasons why you chose not to pursue them still relevant? It's at least worth pausing to think about.
Time to write that book.