One feature that seems (to me) to be highly desirable, yet Discord and every other service like it (that I'm aware of) lacks: the ability for moderators to select several messages in a given channel, and shunt them into a new thread, leaving a link to said thread in their place.
As I've never seen a service provide this ability, it could be that I'm mistaken, and that this wouldn't be as useful in practice as it seems in theory. But, theoretically, this would provide a nice middle ground between forums (and the pseudo-forum-channels Discord supports on Community Servers) and open, IRC-style general discussion. With this feature, users could casually discuss things as usual, but then move to a separate thread once a tangent becomes sufficiently lengthy/heated/etc.