working on the Solution Sketches and after trying to explain the reason of both the Discussions panel and the comments activity filter, I think we HAVE to merge both inside the activity section, and let the user select how they want to "sort" or "group" comments.
Here's my proposal
Document Activity accessory panel header + filters
some notes about the sketch:
we will remove the Discussions button at the top of the accessory panel.
when the user filters the activity by "Comments" a new option appears on the right side of the tabs section, that let the user change the way comments are shown: either chronologically (current way) or by thread (the current discussions panel)
this section of the filters can be activated depending on the parent filtering on the right if needed. In this case, the only filter that needs extra customization is the comments filter.
This way, we can reduce the complexity on users when they need to understand commenting, and also this will be aligned with what Bea is working on for the future