Modernizing is all well and good, but many people use groff to typeset legacy documents. Indeed, one could argue that groff has done an excellent job of walking that line between modernizing and backward compatibility. We have long names, HTML output (however crude), and a lot of people on this list who know more than I do about making it work with corner cases.
Personally, I’d like future development to focus on excellent typography — paragraph-level H&J (which neatroff already has) and hanging punctuation (I’m working on a really crude hack that involves pipelining the intermediate format). Larry