I’ve been looking into @njoseph 's list and I see the technical differences between FB’s pure Debian approach and other ones (container-based or not).
I think that we might find other areas of collaboration.
Would it be feasible to federate hosts so that users of one host might be offered services hosted in other federated host? When the user selects the ‘remote app’ his host should sign a short-lived session-token and redirect the user to the federated host along with the signed token and the federated host would accept the user because the token is signed by a pre-approved host. The signed session token is just one possible implementation, LDAP/oauth-based techniques might offer better choices.
This should greatly enhance the availability of applications reducing the need for personal investments. Providing a service to more users costs less resources and effort than enabling another app. If we make this federation deployment-agnostic our users might jump from FB to YH or others and vice-versa and both (each) project(s) might keep their personality (some oriented to small machines at home, other more open to community/bigger group services, some might provide paid services while others not, some focus more on security while others on privacy, etc).
Security and public reachability are other common areas/concerns that might be fields for collaboration. Basic installation of Debian on SoC hardware, publicity for self-hosting, common presentations, partnering with hardware manufacturers/appliance sellers, performance meassurements, application testing campaigns, etc…
And what’s more important: it would be a starting spark. Once we start inter-project communication it should get more and more fluent, paving the path for further collaborations.