If it’s for money, side projects aren’t worth it. I went down this road for years, and DID make money, but ultimately it’s not worth it.
If it’s to learn, great! Start by solving problems you have. Doesn’t matter if it’s reinventing the wheel, solve it in a way you come up with yourself.
Here’s an example: I do our household budgeting on a spreadsheet, it’s okay but not very user friendly. So I can make a web-tool I can share over local IP so my partner who can’t spreadsheet can contribute. That’s a side project.
Also, do something different to your main job role. I work in web development all day, so my side projects are low level programming graphics projects, or game mods.