I’m not a software engineer by trade so no guarantees, but I’ve been wanting to improve it in some way. I still have yet to understand how Lemmy works, but from using Lemmy I’ve identified two big areas that could use improvement:

Community Tags: Mods being able to tag communities on topics for better aggregation of related communities.

Post Flairs: Users/Mods being able to assign a flair to a post for better client-side filtering of posts, from either a pre-defined list or freeform based on community preference.

For ActivityPub compatibility purposes, either of these could potentially be analogous to Mastodon hashtags, but I still have yet to decide on how that would work especially without it becoming tumblr level.