Here are some things that would be useful additions to groff:

  grap

  -mv macros (for typesetting viewgraphs and slides)

  a deroff that understands long names

  a page-makeup postprocessor and associated macro package
  (like pm and -mpm)

  a complete, self-contained manual -- first results can be found in
  doc/groff.texinfo.

If you want to work on one of these, you should probably post to
gnu.groff.bug to see if anyone else has gotten there first.
