This concept is put forth by Anthony Harmon. He is a branding and UI/app designer in search of a development partner to actualize this concept. If you or if you know someone who might be a good fit please contact him here.
Client side would likely be a web app wrapped in a native installer such as Electron minimizing update deployment headaches and maximizing accessibility across platforms, devices and OS's.
Servers could be hosted locally or on virtual machines and could utilize local (encrypted) or cloud based storage and transfers.
One of the most alluring aspects to Hotline was the sense of community and individuality each server posessed. In addition to the unique file repositories and members at each server, you could further customize aspects of the app itself (news, toolbars, server banner, etc) or by "branding" yourself with a custom user icon.
These qualities are largely missing from contemporary services such as Slack. I propose a constrained, but flexible UI that allows each server to convey it's own brand or identity. Things like custom featured server image, logo, color palette and icon sets would be supported giving each server a unique sense of place.
Additionally, I have worked through several new feature set ideas to further enrich the user experience and and reduce tension points within the app. For a full list of these ideas please contact me directly.