It's interesting how over the past several decades we've learned a lot about which features should be part of a language's standard distribution and which can be farmed off to third-party libraries. String types, standard data structures, package managers, concurrency/async features, type systems, object models, date formats, command-line parsing, log libraries, parsers for the language itself, documentation generators - all of these can be implemented as add-on libraries and have been in at least one language, but we've found that this results in chaos in the language ecosystem. Similarly, protocol parsers, web frameworks, ORMs, command-line utilities (readline/curses, not argparsing), GUI frameworks - all of these have been wrapped into the stdlib of some library or other, but we've found that they aren't essential, and are often better off as a third-party package.

The one feature that IMHO should get baked into a language but nobody's done right yet is API versioning. Wish more people paid attention to the evolution of a program over time rather than a snapshot of where it is now.