Add sticky / pinned topics
Sticky topics are going to be increasingly necessary as traffic increases and speed of submissions does as well, especially for ~tildes topics of site-wide importance like the daily discussions and feature announcements.
Since screen real estate is a limited commodity, especially on mobile, we need a way to limit stickies impact on that. A separate box, independent of the topic sorting, at the top of the front page (for site-wide stickies) and each group, that is able to be hidden is probably the best option. You can also have them automatically unhide again as soon as a new sticky is added to them so people will not forget they hid it and miss important new topics and announcements.