As a background, I am a PhD student in Physics. I have published 3 papers with my supervisor and 3 without my supervisor,as an independent work. After all these, I reached a point where I don't have any good ideas anymore. I don't know what direction I should approach. I have some ideas, some of them are not so good being too simple, and others too complex and might be done in long periods of 2-3 years. the question would be on how to increase productivity and find new good paper ideas?
In my experience a major point is to engage with other's research. The most common occasion is to attend regularly seminars, and go to conferences. By listening to research talks, you will learn about tools which might apply to some of your questions, and about questions for which you might have the right tools.
Another possibility is simply to read other's research, but it is quite difficult to decide in what direction to read, while when you have a weekly seminar to attend you get in touch with ideas regularly without having to be proactive.
Maybe there is a subfield, with connexion with what you already know, where you heard things are happening and someone wrote a nice survey? Then read that and see if it leads you somewhere.