Ability to hide posts you've seen and aren't interesting. I usually either block the user or the community, depending on how uninteresting it is.
Not really possible, but... Have an ability to filter content by region. Removing all US content related to politics and "boring dystopia" would be a happy trade for 80%? less user activity.
A bit of a warning: currently "read posts" also includes your own posts on your profile. If you like to check in on your posts often it can be a bit annoying but otherwise it's a really really useful feature for general lemmy use.