SiSU is fairly mature and for most purposes the syntax and what it is supposed to do is clear. For the most part additions and changes are minor and backward compatible, (in particular there may be things of interest that to be able to achieve will require additions to the syntax).
Amongst the most requested features is a way to represent and extract bibliographies from scholarly and other writings. This involves an extension of sisu markup syntax and a new module to extract the bibliography.
Integration of postgresql tsearch2 / gin indexing, (which currently needs to be done manually, and) which has been waiting for the integration of tsearch2 / gin into Postgresql main, which is supposed to occur in Postgresql 8.3
Internationalisation always. SiSU is utf-8 and for those parts that are utf-8 friendly will work out of the box - html and postgresql for example work out of the box (and for example comfortably represent Chinese text), LaTeX and odf do not work out of the box, they need additional work for extended language sets.
Refinements and improvements to output representations, some are fairly mature, others (such as manpages and info files (and even ODF) remain young.
Simple extension to contain, link and share included audio and multi-media files, (including sisupod.zip)