Since RFC 5424, the IETF have defined syslog's wire encoding and format. However the syslog library calls don't support the extended features (including subsecond timestamps as per bug #11715).
Hence this enhancement request.
Other new features that aren't yet available through libc calls are arbitrary STRUCTURED-DATA and the MSGID element.
I don't know if making these changes would mean a new library call, but I suspect it might.
NetBSD has introduced a new library call, syslogp()
See eg http://www.daemon-systems.org/man/syslogp.3.html
The only reason syslog() as it is exists in libc is because of history. There is no reason whatsoever to have any new version added to glibc. Just provide a standalone version. Just add a library to the syslogd implementation. Coordination is likely in any case needed and this way there are no problems.
The worry about having a separate library is to do with standardisation: if there are rival libraries, then applications will adopt one or the other and the daemon that listens to all this won't know what to accept.
If glibc isn't the right place to do this standardisation, where is?