Complaints will give you great insights. The problem is, it's hard to imagine, ahead of time, the real complaints you'll get.
You can, however, use imagination to see into the future and predict outcomes that will help you shape your idea.
It can give you insight that will either change your idea and product. Or it may reinforce your original thinking and increase your commitment to making it real.
Finish the following two sentences to ignite your imagination:
1. What if...
2. But...
For example:
What if the phone cases were transparent?
But people may want to have all their music with them.
What if the faucet could sense the water is about to run over the sink or bathtub?
But people will think it's silly to wear huge headphones out in the street.
And so on.
Repeat for best results.