Hum, if we want this to be user-controlled outside the plugins (e.g. not have each plugin allow selecting its "priority"), we'll need a way to identify the extensions, like a name. We can add this to register() if need be (more artificial parameters for hypothetical cases 😄), but better add it now that change the API if we know it gotta change.
But are we ever going to add a GUI (or config file) for the configuration?