It could potentially use const-ness for these use cases, though I'm not sure that would be tractable.
More realistically, it could allow the programmer to express it explicitly, as in C# (C<in T, out V> is covariant in T and contravariant in V) or Java (C<? extends T,? super V> is covariant in the first parameter, contravariant in the second). This would not solve the problem that the co-/contra-variance of some types may depend on their const-ness, but it would allow at least a covariant, always read-only, std::vector_view<T> (similar to IEnumerable<in T> in C#, which can also be implemented by the invariant List<T>/std::vector<T>).
More realistically, it could allow the programmer to express it explicitly, as in C# (C<in T, out V> is covariant in T and contravariant in V) or Java (C<? extends T,? super V> is covariant in the first parameter, contravariant in the second). This would not solve the problem that the co-/contra-variance of some types may depend on their const-ness, but it would allow at least a covariant, always read-only, std::vector_view<T> (similar to IEnumerable<in T> in C#, which can also be implemented by the invariant List<T>/std::vector<T>).