Pluggable Rendering Engines?
Flexibility is the obvious advantage. It would allow the author to choose their favorite rendering. A particular use case might be an institution that always used impress.js or Flowtime, this would allow a consistent look for slideshows in the wiki.
There should be a generic way of dividing wikitext pages into slides on section boundaries (and failing that, on div boundaries), then each of those "slides" would be rendered as a sequence within the specified engine.