From what I can tell, accomplishing a more automatic lazy-loading system would require the current file bundling scheme to be amended to support applications written with explicit source file dependency trees.
With the dependency tree, an intelligent lazy-loader could decide which files are necessary for which routes/templates.
In order to not break compatibility with existing applications, the directories used must become configurable.
The default configuration could be described in a file such as .meteor/dirmap.json:
{