Pick someone else's existing work, try to reproduce it, or decide how you would do it differently and try doing that. You'll probably find this leads to more ideas. If not, start again.

On another tangent, there's some good advice in this video:

How To Design Deep Games with Jonathan Blow (Braid, The Witness) https://www.youtube.com/watch?v=d0m0jIzJfiQ

Not just for games developers, worth watching in spite of the terrible audio.