Generating creative ideas that are in that intersection of things that are feasible but yet really innovative, is one of the hardest things we do as researchers. While every research field and every research group is different, from a developmental point of view, I often start my own graduate students on projects where many of the creative ideas are mine; but I definitely hope that they start taking steps toward being capable of coming up with their own interesting ideas in the last year or two years of their Ph.D. study. So, at least in my portion of the world, you are right on schedule to be asking this question: this is the question I want all my third year Ph.D. students to start asking, so good for you.
Let’s start with some context, which is: let’s think about where the project you wrote your thesis proposal on came from. It is probably something that your advisor believes s/he and you are set up to work on, meaning that you have the background, expertise, necessary equipment, etc. If you are being supported on a grant, chances are that your advisor proposed this line of research already to some funding agency, which might mean the piece of generating the initial idea might even have occured several years ago, maybe even prior to your deciding to work with your advisor. If creative ideas came out of whole cloth, this might be cause for despair. However, I have found that usually the best creative ideas come while doing something routine: you have a project that looks straightforward, but then you hit a snag, or the data comes out in a way that is completely strange, or something else happens you just didn’t plan for. In my experience, most of the groundbreaking, creative work happens when you are doing the expected thing, and then hit an obstacle and it sends you off on a new, completely unanticipated direction. I notice that usually, you will hit some obstacle (even if you don’t know in advance what that obstacle will be). And such things are to be seized as opportunities to start being creative. I wish I could give you a recipe for generating great creative ideas, but I think you will find, if you value opportunities to do groundbreaking things, often they will come to you, and often they come in the guise of accidents, but they still come when you embedd yourself in a project and work deeply on it.
However, every field is different, and every advisor is different. This is a great thing to start talking to your advisor about. Ask your advisor exactly this: don’t call yourself uncreative, but ask how s/he generates his/her best creative ideas. How senior was he or she when s/he felt s/he was savvy enough to steer a research project? How can you learn to develop this skill? Most good advisors will be eager to mentor graduate students who ask such important questions, and you might have some very meaningful and deep conversations.