My side projects always come from personal needs, in each case I built it to solve my own problem before turning it in to a full product (summaries below). If you are going in with the sole intention of making money then make sure you know that there is a need for what you want to make and that you can get your project in front of people.

You don't need to use the latest and greatest tech, in fact, I would urge you don't. For front end you can stick to simple JQuery interactions, bootstrap theme and you'll be fine, depending on the market sector you go for they may not even care about the design, so long as it's functional.

Summary of my project and where they came from:

http://www.oneqstn.com, before launching our company's product I put the question "Where would you expect to buy the Radfan?" on the shop page and then 5 options. I expanded this on its own dedicated domain and 5 years later it's still ticking along. Very popular in the middle east for some reason.

http://www.stockcontrollerapp.com, I manage in-house production of my company's hardware product and after moving from an Excel spreadsheet to a Python script I decided to make a stock management app for small factories. Has made my work life much easier and is more appropriate for me than Unleashed.

http://www.taptimerapp.com, I didn't like any of the timer apps that I had tried so made my own mainly for use in the gym. All the others had too small touch targets, hard to see at a distance/without glasses on, or stopped music playing when the timer finished so I made an app that addressed these.