You couldn't live in a better time for being able to pick your work as a Software Engineer. Even location is irrelevant more and more with remote work. If you want to work on something that excites you, find that and do it!
Outside of that, build things that you want to exist using interesting technologies or even just a refined, latest and greatest version of the stack you currently know and use.
Whether you turn these things into actual products or just things you'll use yourself, it's irrelevant, as long as you enjoy building them.
I think that's a bit oversimplified, a lot of really cool projects are simply inaccessible for one reason or another. While there's cool stuff out there, the vast majority available to your average person seems to be the boring stuff