I asked myself that question last November. Big project was finished and finances were good. I applied to YC Startup School to get into the "scene" and they accepted everyone by accident that year.

I ended up doing a startup for backup hosting[1] and a new open source GUI[2] to go with it. Not as grand as the ideas here, but I seems to solve a problem for some people.

If I had to do it again, I would start with a niche problem I know well and that other people are willing to pay for. Then learn new things on the way and expand where it makes sense.

1: https://www.borgbase.com/

2: https://github.com/borgbase/vorta