I think you're approaching this in the wrong way. And, trust me, I have been there and done that over and over again and failed numerous times. People work on a

I think you're approaching this in the wrong way. And, trust me, I have been there and done that over and over again and failed numerous times. People work on a side project for multiple reasons, and give that your reason is simply to augment your primary income, there is only one thing you should be looking for: people who are willing to put down their money to pay for what you want to build for them. Is that easy? No, but there are tonnes of crappy software out there which businesses pay for, and if you replace one of them, you can become ramen profitable pretty soon. What you really need to do is pick an industry (boring the better) that you know well or have good contacts in, and spend some time studying how they work and what you can do to make their life better. You can of course also hypothetically build a product and hope to succeed, but that will be leaving too much to chance.