Related: I would love to see an open source confluence alternative as well, as for issues there are many (more or less capable) options to choose from.
What I most want from a Confluence alternative, and I have a few poor ideas how this can be solved, is a way to manage stale content. The Confluence spaces I work with are dumping grounds of old, often misleading, information.
Deduplication guidance, reports on untouched content…I don’t know how to fix this, but someone surely can solve it.