How to find something to work on for myself?

I feel absolutely stuck. I'm employed as a founding engineer at a small start up, I've created the vast majority of the technology and stack that generates millions in revenue a year.

Yet I can't think of anything to work on myself, or where to apply my efforts for my own thing. I read of people saying 'target what you know' but I don't find it helpful. I don't really have much friction in my day job. I don't want to create a super-spread sheet or yet another dev tool.

I don't want to try to monetize my interests outside of work (reading, art).

I look for ideas off of what is posted on Acquire.com but inspiration doesn't strike. I'm scared of dedicating a lot of time to an idea I don't feel great about only for it to not make money. What would motivate me is a clear path towards making money. If I was confident that whatever I build could be sold to customers, then I could figure out the in-between steps.

I just feel like I'm stuck at step 0 on trying to find something to build and I'm stuck working a job, that while it pays well, will never give me financial freedom.

Just wondering if anyone has felt this way and has any advice..