* Listen on specific interfaces or protocols (eg. only IPv6).

* syslog? journal?

* Performance - measure and improve it.

* Bash tab completion.

* filters: It should be possible to layer filters over plugins to do
  things like injecting errors and adding delays (see wdelay/rdelay
  options in the file plugin).  This could also be used to implement a
  "copy-on-write" feature popular in other servers.

* Factor out the nbd client in the nbd plugin and make it available
  to other consumers, so that nbdkit can be layered.

* Suggestions for plugins to be written:

  - glance

  - cinder

  - XVA files
    https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg02971.html
    is a partial solution but it needs cleaning up.

  - libssh

  - libnfs

  - iscsi

  - gluster

  - ceph/rbd
