How do I find “up-and-coming” open source projects to work on?

There's tons of really cool open source projects out there that I'd love to work on, but the bittersweet truth is that many of them are quite mature, meaning the pace of development has often slowed somewhat, and there's often quite a lot of theory and domain-specific knowledge to "get caught up on" (which, I will admit, is quite intimidating, especially since I don't want to waste people's time by asking too many questions).

I've spent some time working on these large projects (various libraries and tools, a brief foray into the rust compiler), but it often feels like the only work I'm able to do amounts to just small ad-hoc bugfixes and refractoring, which while I understand its importance, doesn't really "feel" satisfying.

Ideally, I'd love to find a cool project that's far enough along that the owner/maintainers are accepting contributions, but still early enough that the whole project can be kept in your head pretty easily, and still requires significant development. Does anyone know a good place to find these projects? (perhaps requests for contributions or something?)

Of course, one could always say, "why not just make that project yourself", but sadly coming up with an original, impactful idea is quite difficult. I've played around with everything from making parser generators to 3d games, but they're just solo projects that always end up as pale imitations of other projects - fun to make, but not impactful at all.