The go plugin ergonomics are terrible, having to have exactly the same hash of every dep and compiler as the server is terrible unless you are compiling at the same time. This is my experience with the custom tokenizers in Dgraph which already use go plugins.
But WASM allows everything you said: loading on demand, etc. And then you would not have to worry about syncing dependencies and could write it in any language you want, including lua (embedded in go,rust, or cxx)