It would be useful to support reStructuredText as the wiki markup language, probably with some small extension to make wiki page linking easier.
The main reason for this is so that the wiki and README files use the same markup language. This will allow the README to be synced with a wiki page if desired (this is desired for one of my projects). Also, other documentation written in reST can be synced with the wiki. I imagine that this would be a repository option to make the wiki use reST rather than Creole, or per-page modeline detection (if the wiki page starts with an Emacs or Vim modeline indicating reStructuredText, render it as reST).