DocBook support:
olink (do it like what the DocBook XSL stylesheets do)
callout and friends. This is the only thing that prevents error-free (but not warnings-free) conversion to Texinfo of DocBook: The Definitive Guide.
The qandaset support now sucks.
For other work items, see the “limitations” or “bugs” section in the individual tools' reference pages.
Stylesheet reference documentation.
Figure out some way for manuals not written in a strict reference style (like this one, and most Texinfo-based manuals) to be condensed or summarized automatically into man pages.
Fault-tolerant, complete error handling.
Rigorous tests, particularly for lesser-used DocBook markup such as tables.
Here are some features that I have considered adding to docbook2X but should not be:
Make conversion of SGML documents to XML automatic.
This would be a general tool and should not be docbook2X-specific. Anyway for automatic man-page and Texinfo generation, sgml2xml with the correct options does most of the job.
Let the Texinfo XSLT stylesheet automatically work on any supported processor.
It is difficult, since XSLT does not support the method of “conditional inclusion”, which is what is used right now. Anyway using docbook2X on Java processors out-of-the-box is not such a sufficient concern.