If I were to pick one “next big feature” request for the Meteor Guide (and documents too), it would be user sourced annotation support.
The guide should and does have a fairly concise “do this and it will work” approach to giving advice. Often, though, it would be nice to be able to easily dig deeper and see some of the justifications / reasons. Being able to turn on an annotated view that perhaps presents the annotations in a right side pane and allows you to click on a larger annotation to go to a annotation only view of that particular one would be awesome.