The ONLY lib missing from stdlib. Is the time library.

Maybe unified interface to work with DB.

Cryptographic functions while useful age poorly, and require constant change.

Serde while extremely useful won't fit every usecase. Sometimes you want something smaller or faster.

Http(s) clients also tend to age poorly. Python also had http client/server in std lib that aged like warm milk.

For a thing to be in std, it needs to be imo something you don't want implented in several different ways. Something that is essentially solved and will change very little over time.