The sprites should be generated so that there is one sprite with all icons for the main forum index, and sprite files for subforum categories. i.e. if a forum holds 10 subforums, then those 10 node icons should be on 1 sprite, because the reader will load those icons only if the reader accesses that forum.
My big board with hundreds of nodes caters to millions of readers, so the addon should be very well coded and not incur performance issues / page loading delays. Sprites will be loaded from a cookieless domain.
I would like the addon to be publicly released for the XenForo community to use. (free or commercially)
I need this functionality to be developed within the near future, so please respond if your time schedule allows for this project.