Interfaces are the most important thing Zig needs to implement ASAP
It would also help creating a uniform standard library, for example creating a Writer interface instead of having specific functions for files, sockets, buffers and all kind of types
For the equivalent thing (interfaces with runtime dispatch and without owning the underlying struct) Zig has explicit vtables. I sort of think any new "interfaces" feature should at least partly be about describing constraints on types at compile time.
It would also help creating a uniform standard library, for example creating a Writer interface instead of having specific functions for files, sockets, buffers and all kind of types