Simple answer: Anywhere you'd use C, and want a nicer language. So: embedded, operating systems, drivers, compilers, PC applications etc.
I would love to try it out with a serious project, but am waiting on libs like HALs for microcontrollers, GPU API bindings, GUIs etc to mature to a usable point.
Well, I use C++ for that today and don't see much benefit in switching really. Switching to a memory-safe language is something that I can support and even sell to my team, but switching to just a "simpler" language I'm not sure...
The Zig stdlib and comptime features like generics and type inspection go way beyond C (and in parts also beyond C++ and Rust) though (e.g. Zig is much more than "just" a C replacement).
I would love to try it out with a serious project, but am waiting on libs like HALs for microcontrollers, GPU API bindings, GUIs etc to mature to a usable point.