Douglas Englebart must be rolling in his grave at a comment like this https://www.youtube.com/watch?v=yJDv-zdhzMY
It surprises me that Confluence continues to be the only thing sitting in the middle of those, and while some competitors have started to shyly emerge (Bookstack, xWiki, I think there's one in progress at Jetbrains), nothing feels like it's really aggressively going after the Confluence market-space directly. Given how many people I've seen comment that they're not fond of Confluence but can't seem to replace it (at least, not until Atlassian forced their hand with the server-license fiasco), it seems like a natural space for people to pursue.
Send me an email at Paul at dokkument.com and that's exactly what we are working on. Yeah unforturnely you will not see anything interesting on our landing page