Some things I have in mind, though it's very unsure when and if ever:
Anything related to the markup should be put into docutils -- I think wiki links are nearly all we need in the way of extensions; everything else can be done in docutils directly. (Though perhaps there should be a general directive, which simply passes parameters to the Wiki to be rendered at display time)
Put whatever you want here.
Possibility to generate static pages for fast and scalable read-only with any server. request granted
Breadcrumb links (showing your recent path through the wiki)
IB: What's the value over the back button?
CG: Well, if you click back then click a link, it erases the forward history, so it's handy for that. I don't think I described it right -- it's more of a static-length page history with most recently visited pages at the top (and when you visit a page in that history, it pushes it back up to the top). It lets you do non-linear history traversal. :) It's not really THAT useful, but MoinMoin has it, and I find that it comes in handy when you want to go back and forth between two wikipages that aren't linked directly to each other. Plus, it's prominently visible, so you can quickly jump back 5 pages with ease!
Petition the docutils guys to make their formatting warning messages include a piece (or the whole) line that generated the error. ;)
Javascript function that clears the "click here to search..." field when you select it. (IB: it's supposed to do that now; works for me on Mozilla at least)
ReST Quick-Reference link on editing page (popup window)
Things I don't really have in mind, but maybe would be a good idea: