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

 ideal preprocessor

 -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)

If you want to work on one of these, you should probably post to
gnu.groff.bug (or send email to [email protected]) to see if anyone else
has gotten there first.