Perhaps a 3rd version of mosquitto would be useful, one that targets high performance and multi-instance horizontal scalability using a pluggable shared database for retention and a pluggable message bus for distribution.
In my experience, this would be much easier if you first make the core simpler: e.g. remove all unneeded features, such as bridging; and remove as many options as possible, e.g. always incorporate TLS.
Code that is smaller and easier to read will draw more eyeballs.
Then I would keep the core simple by using a micro-service approach to plugins, interfacing via small "hooks" and allowing them to be implemented in any language - I have done this using ZMQ in my POC but there are other approaches.
Core simplicity also lends itself to event loop implementation. We use libev but there are other choices. It is possible to make the event loop pluggable with thin abstractions.
Anyway, those are my thoughts.
Michael
