Features I really wish notmuch supported:

  * Public access to message and directory Xapian::docids, which could
    be achieved by exposing the _notmuch_message_get_doc_id and
    _notmuch_directory_get_document_id functions.

  * A Xapian value attached to each document that represents the time
    of last modification (i.e., filenames added or tags added).  This
    would allow super fast incremental message syncs.

  * The ability to use glob patterns or regular expressions for
    user.other_email, e.g., for situations where one receives mail at
    <anything>@some.domain.com.

  * Indexing of Delivered-To headers, since this is often the only
    exact (non-heuristic) way of knowing that you received a
    particular message from a mailing list.

  * A more precise folder query.  (For example, how to match against a
    top level folder, to find messages in inbox/cur but not
    inbox/anything/cur.)

  * A way to re-send bounce messages, editing the headers to fix
    incorrect addresses, which most mailers support.

  * An out-of-the-box feature to stick a Return-Path: header in
    messages saved directly to the sent folder.

  * Multithreaded message indexing, or at least the ability to merge
    multiple databases so the expensive indexing part can be spread
    over multiple CPUs.  (This is quite hard given the current way
    that thread IDs are created.)

  * Better handling of non-identical messages with identical
    Message-Ids.  (This is really a design problem, so unlikely to get
    fixed... it might be better to use content hash instead of the
    insecure Message-ID header.)

  * An option to ignore files or directories that don't fit certain
    patterns, especially anything that doesn't end with .../new or
    .../cur.  notmuch seems to waste an inordinate amount of time
    looking at weird databases and other crud left around by imap
    servers, and explicitly listing each individual file name in
    new.ignore is a pain.  At least glob patterns would help, but what
    you really want is a list of allowed directory names (i.e., new
    and cur), not ignored files/directories.