It’s hard to commit yourself to a thesis topic. You don’t yet know if the idea you have now is a good idea or not, and if you commit to it, you might not be able to pursue a better idea you could come up with later.
So here are a couple of guidelines to stop you going round in circles.
1: Stop looking for the one big idea
Start investigating something… anything at all, and see if it grabs your interest.
This might not be what you end up researching, but just exploring your subject can lead to unexpected flashes of inspiration. You might try 3 or 4 (or more) ideas before you hit gold.
2: Talk to people
Share ideas with people in your department, and see what they think.
But also find out what other people are doing. There may be a what’s hot in your subject at the moment. Are there big gaps where nobody else is looking? Should people be looking there?
3: What do you want to learn?
A PhD isn’t just about your contribution. It’s also about what you learn, and the skills you can use later in your career.
What skills and knowledge do you want? What are you already good at and want to improve?
4: What do you like doing?
Choosing your research topic is choosing what you’re going to be doing every day for the next few years.
What do you enjoy the most? What do really dislike doing?
5: Combine ideas
Once you’ve followed the above steps, if you are still stuck but have a few ideas to play with, try combining ideas in new ways. Applying technique A to area B.
There are very few truly original ideas, but there are always new perspectives on old problems.
When you find a combination that could be interesting, useful, or even exciting, go for it!