Ask HN: Need advice – I can't decide on a side project to work on

I have a full-time job as a software engineer. I'm in my 20s, and my eventual goal is to create something of my own (a software project) on the side which can earn me money, so that I can focus on it full-time and quit my full-time job.

When selecting a project I focus on the following criteria: 1. There is an actual need for it - Since it's something that I want to bring me decent income one day, it should be marketable. I can't just create something and hope people will like it. Also, not in an over-saturated market. The world doesn't really need another dating app, or another todo-list app. 2. It is fun to implement - It should be exciting to work on. For example, finance-related software can be a money-maker, but I find it utterly boring. I don't want to dread working on my own side project. 3. I can work on it on my own - I want it to be something I can create the foundations of on my own - not something I need partners from the start. I would only involve more people when the project reaches maturity.

All of the ideas I've had so far cover only 2 of the 3 criteria.

When I start working on a project that covers only 1 or 2 of the 3 criteria, I immediately give up on it and start searching for another one.

My current job involves work on Android, but it's not something I'm particularly passioinate, so that fails criteria 2.

In terms of my interests, there's entertainment (movies and video games), healthy living (workouts, gym, diets), travelling, educational videos, MOOCs, 3D printing, 3D graphics. I believe all of those are over-saturated in terms of software solutions, but I'm happy to hear your thoughts on this.

I've also read several past posts on HN on the same topic, including PG's essays. But I just can't notice any obvious problem that is not already solved.

Is there any advice you could give me? Or a framework I should follow? Or perhaps ideas that come to mind?