Recently came across a very interesting federated social network and “web-ring” kind of application called HubZilla: https://project.hubzilla.org
It is a sort of CMS / federated network that allows nomadic identities (https://project.hubzilla.org/wiki/hubzilla/Feature%2BArticles/Nomadic%2Bidentity ) and a whole lot of unique other features. An good overview can be found here:
And for me personally the coolest is that it integrates quite seamlessly with my Nextcloud as it also supports Webdav for files and CalDav for calendars. It also uses a standard LAMP server, so setup is very similar to Nextcloud.
I recommend you have a look at it and maybe someone could think of a way to link these two PHP apps even better?
HubZilla has a really advanced SSO solution build in (see: https://macgirvin.com/wiki/mike/OpenWebAuth/Home ) so if Nextcloud had support for that, it would probably allow interesting direct interaction with various HubZilla instances.
Looking forward to what you think about it!