The left-swipe navigation drawer on android is a well-established standard. Google has detailed design guidelines here:
However, they typically contain many far fewer items than Discourse’s hamburger menu. ie. <10 so that no scrolling is required. Slack have just changed their interface, but the previous version had a nav drawer for channels, contacts and groups.
The current meta.discourse hamburger has 30 items, which is more than I have in my Slack nav drawer (~20). It’s an uncontrollable number of course, since people can run forums with 100 categories if they want. On my site, it’s only 14, but that’s without including sub-categories (which I’d like to see, as it happens).
+1 for consolidation of Notifications and avatar menu. It would be very clear if the hamburger on the left was for “places” and the avatar on the right was for “me” Again, this is how the previous version of the Slack app did things. (although they had no dedicated notifications area, just (1) next to channels/people who’d @mentioned you)
I think @erlend_sh’s mockup gets it just right (although I’d keep search as a glyph rather than a text box, to conserve space on mobile)