I would like to suggest the addition to the standard library of a package to generate and parse UUID identifiers, specifically versions 3, 4 and 5.
The main reason I see to include it is that the most popular 3rd-party package (github.com/google/uuid) is a staple import in every server/db based Go program, as confirmed by a quick Github code search.
Additionally:
UUID is a standard;
The interface exposed by github.com/google/uuid has been stable for years.
Addendum
Would like to point out how Go is rather the exception than the norm with regards to including UUID support in its standard library.