While the Sidebar certainly doesn't have m(any) of the functions described in the OP, a basic sidebar has been added for 4.0.0 Alpha 1. So far it contains the Wiki Activity Feed, the contents of the full wiki book when viewing one of its chapters, and we have quite a few more planned. NickyPhils should be joining us later in the week to assist in writing algorithms for some unique article metrics. Using Javascript, we have gotten the article's content blocks to wrap around the bottom of the sidebar nicely, and it is also possible to show/hide the sidebar.
A Sidebar Manager for adding custom blocks can come later. It's already possible to add custom blocks using PHP code by creating plugins that run your plug_blocks method. Simpler HTML-only blocks would have to wait for a manager unless you have the know-how to make PHP spit out the HTML. In the interim, there will be AdminCP settings to turn individual blocks (or the entire sidebar) on and off.
From this point on, individual sidebar blocks and the manager should be tracked in their own separate threads. Leaving open until the aforementioned settings are added.