we use our github issues - a lot of users have great ideas and they share it with us
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Listening to your customers is better if you already have a product, but focusing on your vision is more important.
If not, it's better to solve a problem you faced or others faced worth solving. Otherwise, dive deep into your industry or exciting topics you will find a problem worth solving.
1. spend time researching your competition.
2. reach out to your customers and try having phone calls with them.
3. analyze your data
4. talk with your team
5. talk to enthusiast people who would love to give you feedback and new ideas (crowdsource).
twitter gets tons of feedback for new features for example
Once you've got your ideas, I have found that Y Combinator has a some really great YouTube videos from their past Startup School sessions about Evaluating Startup Ideas. I would highly recommend these videos!
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Epicmax
Twinr - App Builder
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JinbuPal