Very often when searching, you would want to search for email addresses. Allow the user to use the configured address-book-cmd to complete email addresses. For maildir/imap
:filter it might be enough to add completions to the appropriate flages.
For notmuch it's a bit more complicated for two reasons:
:cf to:(email@example.com OR firstname.lastname@example.org)
One solution to this could be a configurable bind to trigger address completion, like the built-in vim completions. Another could be trying to be really smart to figure out when to complete. The former might be conceptually much easier, the latter might pave the way for further completion support for other notmuch search operators, e.g. mimetype, path and folder.