For those seeking out a federated and open alternative to Tumblr, Wafrn is looking extremely promising. It can speak both ActivityPub and AT Protocol, offers a ton of interesting features and customization, and focuses on making an incredibly fun experience.
Reddit has that, and the ability to follow a user and get notifications when they post. I’m not sure it’s widely used there, but I think it would be a decent feature to add to Lemmy.