The new table of contents plugin for Wiki pages is nice, but not too useful in a Wiki that strives to keep the content of each article fairly "atomic". I am creating a new product spec in the FogBugz Wiki, where each article is either an introduction to a group of features or a feature description. there's not a lot of hierarchy in each article, but there is in the overall Wiki.
The ability to generate a table of contents with links to the entire wiki would be great.
The ability to insert (i.e. insert while serving) that table of contents into various articles would be better.
The ability to insert that table of contents and have it accordion close all but first level articles, automatically accordion open to show the current page and to highlight link to the current page as a "you are here" indicator would be sick/sweet/slick/cool/neato.