I wouldn't start a new wiki engine with the same features that everybody has, there are so many of them out there.
I would only work on one that offers something different/unique, much more than a standard wiki.
Some ideas include (maybe some wikis already have this):
- The ability to have the wiki merged with a VCS, to discuss revisions/changes and be able to do code reviews (anything that gets committed automatically creates a page for the revision), having it linked to the committer and anybody in the discussion and sending them email alerts would be nice.
- An API for the wiki that allows 3rd party application to do mashup-style integration.
- wiki-style multimedia (Text, Images, Audio, and Video) that contains links to other media.
- multilingual side-by side editing and translation.
- a client-side editor/viewer (not web based) for faster response and real WYSIWYG editing.
- ...
