I also have another idea for a favorites selection tools. It should look like the selection tool but where you find current selections you should see your favorites dimensions and fields. I also would like to be able to set app, current sheet and custom groups favorites. Those would be picked from a toolbar at the top.
as of now we only have "Visualization Extensions" but having other extension types (like e.g. document extensions in QlikView) we might consider in the future.
Beside your idea of having a calendar selection tool & a favorites selection tool I am always keen on understanding different scenarios where other extension types really help. In QlikView we had the document extensions but to be honest I have not seen a lot of solutions out there using this extension type, most of the extensions I have seen were based on "object extensions".