[mpercy]

Since I mentioned performance, one other area that makes flexibility nontrivial is how you decide to serialize different types of messages on the wire and on disk. If you don't need extensibility, it's easy to keep things pretty efficient just by using e.g. gRPC and protobuf out of the box. If you want complete flexibility, the simplest thing to do is to give your plugin interfaces blobs to write into and you end up double-serializing everything.