I'm again toying around with the idea of building an ActivityPub Server built around the principles of RDF, JSON-LD and the Linked Data Platform. [0]

It can work already as a "Generic" ActivityPub server and it can be made to work with Client-to-Server API, but given that there are not mature clients for that, I am now in the middle of an exercise where I am taking the existing server and implementing Lemmy's and Mastodon's APIs based on top of it. Once I can get any Lemmy and a Mastodon client working, I will then start changing their own SDKs, and then I can replace calls from their application-specific APIs with direct calls to Linked Data server.

  [0] https://activitypub.mushroomlabs.com