That's great! Is there a reference to what data is always included separate from that specified by the created data elements?
Not sure if there is a specification about what variables are set automatically. It will be challenging to prepare such a guidance as the list of variables may vary depending on library configuration, plug-ins, onsite events.
You may want to read this page, the chapter Pre-configured reports in Reports & Analytics explains what gets available out of the box. It not only covers the data captured in variables that you see in the debugger but also the info from http headers (e.g. user-agent you were referring to).