The following is a list of things I would like to see
done with libarchive.  It's sorted roughly in priority;
more feasible and more often-requested items are higher
on the list.  If you think you have time to work on any
of these, please let me know.

* More compression options:  Recent improvements to the
  read bidding system and external program support should
  make it very simple to add support for lzo, lzf, and
  many other command-line decompression programs.
  I've even written up a Wiki page describing how to
  do this.

* cpio front-end.  The basic bsdcpio front-end is now
  working.  I'm looking for feedback about what additional
  features are necessary.

* pax front-end.  Once cpio is a little more stable, I plan
  to fork it as the basis of a pax front-end.  The only really
  tricky part will be implementing the header-editing features
  from POSIX 2001 'pax', which will require some changes to
  the libarchive API.

* libarchive on Windows.  libarchive mostly builds cleanly
  on Windows and Visual Studio.  Making this really clean
  requires reworking the public API to not use dev/ino; I
  think I know how to do this but could use advice from
  someone knowledgable about Windows file-management APIs.

* Linux large-file/small-file dance.  libarchive always
  builds with 64-bit off_t and stat structures; client programs
  don't always do this.  Supporting client programs built
  with 32-bit off_t requires a little trickery.  I know how
  to do this but haven't had time to work through it.

* bsdtar on Windows.  After libarchive is working on Windows,
  this should be much simpler.  At worst, you can just disable
  features.

* Writing tar sparse entries.  The GNU "1.0" sparse format
  sucks a lot less than the old GNU sparse format, so I'm finally
  dropping my objections to sparse file writing.  This requires
  extending archive_entry to support a block list, and will
  require extensive changes to bsdtar to generate block lists.
  The sparse read code will also need to put block lists into
  the entry so that archive-to-archive copies preserve sparseness.

* ISO9660 writing.  Writing ISO9660 requires two passes, and
  libarchive is a single-pass API.  For ISO9660, you can hide
  that behind a temp file, though.  Collect metadata in memory,
  append file bodies (properly padded to 2k sector boundaries)
  to a temp file, then format the directory section and copy
  the file data through at format close.

* archive_read_disk:  Currently, libarchive can generate a stream
  of entries from an archive file and can feed entries to an
  archive file or a directory.  The missing corner is pulling
  entries from a directory.  With that, libarchive can provide
  efficient bulk copy services for dir-to-dir, dir-to-archive,
  archive-to-dir, and archive-to-archive.  Right now, the
  read-from-disk capabilities are handled in the client.

* ISO9660 Level 3.  ISO9660 Level 3 supports files over 4GB.

* --split=<limit> option to bsdtar.  This would watch the total output
  size and begin a new archive file whenever <next file size> +
  <total archive size> exceeded <limit>.  Not as robust as
  GNU tar's ability to split an entry across archives, but still
  useful in many situations.

* Filename matching extensions:  ^ to anchor a pattern to the
  beginning of the file, [!...] negated character classes, etc.