Plugin support for tools
The Python libraries are useful for building new tools, but there's currently no good way to extend the behavior of an existing tool without copy-pasting code. There might be opportunities for plugin-like extensions. For example, luafmt/luamin could be given the path to a LuaWriter class that is run in place of the usual writer or in addition as a post-processor.

Feedback welcome on what kinds of plugin APIs would be useful.