Make remote community discovery more intuitive.
Make remote community subscription more reliable.
Make navigation & reading work without javascript.
Make dark mode available when not logged in.
Indicate which comments are new when returning to a recently-visited post. (old.reddit.com does this if you have premium.)
Display user and community names with the domain part dimmed (and maybe on a separate line), for less visual clutter at a quick glance.
Display user names without prepending an @ sign, for the same reason.
Allow sorting community lists by name.
Horizontally align all community names in lists, regardless of whether they have icons.
Reduce wasted screen space.
Optionally hide avatars & community icons.
Optionally (admin choice) mirror remote instances' images, so they can't be abused by remote parties to track local users.
Optionally (user choice) disable or replace remote images, for the same reason.
Stop auto-inserting new items into a list that's being viewed. (It causes what I'm reading to suddenly shift or disappear off-screen, which is disorienting.)
Make buttons work reliably. (Clicking them sometimes applies a border without doing anything else.)