On a personal level, pick something to work on at the intersection of a personal cause, where you can create market value and you are good at.
When you have the opportunity to start something new, choose something the founding team personally cares about and can commit to for a long time. Convincing other people – and sometimes even yourself – that doing this new thing is a good idea is much easier when passionate about it. Avoid blindly following a passion, though, and instead look for what keeps coming back to you yet is broken for you and other people.
Looking for what is missing in the world and where you can create market value is the essence of entrepreneurship. Knowing the market means having expertise in the problem. You can have such expertise from your industry experience or acquire it by immersing yourself in a new domain hands-on for a few months. The best problem expertise is having experienced the problem yourself because most likely others have too.
If you value personal craftsmanship, pick something you are good at, a field in which you have solution expertise. Ideally, the problem is hard, but you are well suited to solve it in a way that is substantially better than existing alternatives. Ensure that the founding team complementary has all the skills needed to build the first version of the solution.