If I had a very generous sponsor to cover the huge time commitment required for this, I would fork an #ActivityPub server (probably #Mastodon or #Firefish) and:
Rearchitect it to use industry-standard products for queueing, streaming, relational, key-value, and in-memory stores. Ideally everything must be so widely used that a major cloud provider offers a managed version of it.
Standardize the backend on one (1) technology wherever possible. Probably Python.
Making it as easy as possible to deploy on your own infrastructure or leverage managed cloud services. Also easy for the wider community to contribute. That's the dream!