What I do is I look at the products I absolutely love. Then I think about if the concept can be applied to any other industry.

Think Uber for... (But seriously, don't do 'the Uber of [anything]'.

My latest side project I was thinking about a product I love and I thought about how I use Buffer for social media management. They have made my life, so much easier! From there I thought about if the concept could be applied to any other industry. I realized the messaging industry had nothing like it! Needless to say, I am not working on 'the Buffer of messaging', but that is where the idea spawned and I refined it from there.