Recently I've been trying to implement the ActivityPub protocol. I wanted to get more of an understanding of what the issues are with it, and see if I could implement a server from scratch. Mastodon is ok, but too resource intensive for my use cases. The filtering system of Pleroma generally works well, but I was still struggling to keep bandits out of my inbox and it was becoming too much of a chore. Self-hosting is supposed to require little to no maintenance if it's done right.
If I'm to remain in the fediverse at all then what I'm looking for is something which requires minimum RAM and storage space. Where the database size has a strict maximum upper bound. And where I can be confident about what (or who) is or isn't getting onto my server. I searched around for existing projects which might fit the bill, other than Mastodon or Pleroma. GNU Social and PostActiv are still around and they were a good solution a few years ago. But I think the state of the art has moved on and something like GNU Social isn't geared up to handle the adversarial situations which now exist. It was designed for a gentler world of Free Software developers exchanging cycling trip photos and commandline tips. Now that there are a million or more fediverse users it's a different game entirely and the blooming buzzing confusion of the crowd requires some taming to be humanly interpretable.
So I may spend the next period of time developing a minimal fediverse server, equivalent to an email MTA. Maybe it won't work out and there will be some show-stopping reason why this is a bad idea, but in principle it seems like a tractable piece of work. On top of all the usual features it would also be interesting to experiment with adding organizing features and also something comparable to the old GNU Social Sharings plugin for bartering and freecycling.
I have some initial code here. Of course, it had to be named after a species of extinct megafauna. It's highly experimental and mostly just a bunch of unit tests, so I don't recommend that anyone use this for any practical purpose right now.
In case you were wondering, the next version of Freedombone will be out soon although I don't expect it will have any fediverse servers. In my estimation the existing software is too unsafe and too high maintenance for an install-and-forget type of system.