I know this message thread is now over a year old -- but it discusses the exact reason I found myself here.
I recently got NextCloud 16 up and running in an iocage jail on my FreeNAS server at home, and I've configured it so my friends can sign in and create their own user accounts (via a registration plug-in for NextCloud). Then, I modify their access levels to suit to grant them access to several shared folders I've set up, and have a chat plug-in installed so they can chat with each other when they're online simultaneously. It feels like the big item it's missing is a message forum, though. And from everything I read on NextCloud's help site, there's really no developer interest in building one into it. (Very disappointing, IMO.)
I'm struggling a bit with any kind of forum software that has to run in a separate virtual machine from my NextCloud, simply because I'm running on a broadband connection with only one IP address (no static IPs). Can't afford to cloud host forum software externally though, for just a personal project like this.