You need to have a solution to a problem people are willing to pay money for. You are not likely to find that by asking on YC. Your best bet is to pick a long

You need to have a solution to a problem people are willing to pay money for. You are not likely to find that by asking on YC. Your best bet is to pick a long term software developer contract at a large company. Are you looking for anything you'd pay for? I'm fairly certain that anyone here with an answer to that question would be working on monetizing it. If you want to find an answer, pick and industry (preferably one you have experience in), find out where the business owners hang out, then ask them what problems they have.