Not much that wouldn’t also kill them, I think.
Reddit has become too massive for its own good, and it lost its sense of community from the early years. There was a few nice subs, but they usually ended up being popular for exactly this reason, and they ended up being connected to the “big centralized Reddit bubble” (if that makes sense), which killed the community in the process.
My best memories of fun or interesting conversations on Reddit were actually not made on particular subreddits, but more on recurrent stickied threads on some subreddits that only a few regulars opened and read. Those had a real sense of community.
So yeah, Reddit lost me as a user these past few days, but not 100% because of the actual changes that they made - I think I was already dissatisfied with it and that was just the straw that broke the camel’s back. It’s more like a combination of the massive user base and the way the website works that kind of suffocated communities. They cannot really change that, as they would probably not survive changes that are too big or a drastic reduction in the user base.
The Fediverse could suffer from the same issues if it becomes wildly successful of course, but the fact that it is federated adds another layer of separation between community circles, and I think that’s enough for mitigating that problem a little bit.