Ideas are a peculiar thing, I’m not really sure how they happen - I’m not sure anyone does. The nature of ideas has always fascinated me as well because they always come out of nowhere. What I think helps the most, at least for me, is to keep an idea journal. As an artist, I go through big ups and downs with ideas. So, I make sure that I write them down when they come up, and I write down everything, because you never know when it will come in handy later.
Also, be open for inspiration from any avenue, but find things you like, and build from there. Personally, I love cinema, so a lot of my ideas have stories behind them. I am planning on shooting one of my scripts I wrote a while back, but with photos, when I am done with my 365. I am really excited about that. Also, I love period Americana, so a lot of my images have that aesthetic, ie tweeds, solid colors, chambray, work-shirts, dark blues, greens, greys, hats etc. I think once you start to identify a base of your own interests, ideas will start to come out of that. Also, I find, the more you shoot, the more ideas that will start to come, because you find different ways to do things.
Conceptual photography is great. It is a ton of fun and is really creative.