It would be nice to be able to assign an icon (or emoji) to a channel so that it replaces the existing public/private icons in the sidebar. Having a small image that describes the channel allows for people to more easily find a channel that they are looking for in a large list of channels (just like it does for DMs and teams). The space taken up by the existing public/private icons would be better served by a custom image/emoji. If desired, the public/private icons could be retained by making them small overlay icons in the corner much like user statuses on DMs.