One of the strongest differentiators of this library is (arguably! (I'm arguing)) how declarative it is. Ensuring the primary usage of the library is via struct literals will support this differentiation, imho.

In practice, this can certainly become awkward to the point of being a bad user experience. For example, consider the "value source" implementation Sources: EnvVars(...) that replaces the v2 EnvVars: []string{...}. The EnvVars function is not a literal declaration of a type, but the usage is expected to be within a literal declaration of a Flag type.

Judgement calls must be made!