As similarly mentioned by others here, I think the best ideas for my side projects come from the deepest frustrations.

Not so long ago I created https://goswitchboard.io/ as a way to automatically organise my downloads and movie folders on my machines.

I've since made it into a pro version which I've had a few sales from, nothing considerable, but enough to pay a small monthly hosting bill.

Alas, find something that irks you in your day to day development or work and build for it :)