I would like to propose that the search order of the <master> tag (when the "src" parameter is not specified) be as follows:
* search for a master template in the current directory (ie, directory-local master)
* if none found, search for a master template in the package's www/ directory (ie, package-local master)
* if none found, use the subsite master template
* special case: if a page is itself a master template and invokes "<master>", then it should obviously ignore itself and begin searching one-level down
* the names of the local masters would be specified as subsite parameters, but would default to "default-master"
Now that we've removed the package-wide masters in most, if not all, of the current packages, this proposal would allow us to regain the flexibility of adding customizations at the directory and package levels. Examples of this can be found in the aforementioned thread.
I'm willing to implement this for the 4.7 release.