When the time comes, I'd say a new side project (like bbPress) should be WikiPress.
Easy integration with WordPress and bbPress, similar plugin/theme/admin structure, simpler than MediaWiki and yet as deep as WordPress.
Yeah, it might be hard to imagine a wiki that's not MediaWiki. Still, a few years ago it was hard to imagine a blog service that wasn't MovableType.
And last but not least - most wiki solutions out there today is kinda... strict. I'd like mine to be as easy to extend and modify as my WordPress installation - with the ability to quickly make it look and feel like my blog/forum.