I started writing this for the Improving the Internet General thread >>34736, but it probably deserves it's own thread because it goes into much more detail about a specific sub-topic. Some of the messageboards mentioned in the alt-chan discussion >>>/r/14350 have some interesting features/ideas. Off the top of my head---there are definitely more, if someone's willing to go through all the messageboards listed there---I recall (sorry about all the abbreviations!):
sHiTMyseLf / hike https://github.com/qvtqht/soykafmyself is a semi-anonymous messageboard. You can write anonymously, but it's integrated with PGP identities so you can also write under a persistent pseudonym which introduces optional, easy to shed reputation that's not dependent on the forum.
RockSolidBBS / RetroBBS / NovaBBS https://www.rocksolidbbs.com/ https://www.novabbs.com/ federates via the granddaddy federated messageboard protocol NNTP / Usenet, has a web-interface, an I2P `eepsite,' and I think they have a TOR `onion' service, but I might be confounding it with NNTPchan. Or they merged?
cursedboard / whisperchan https://github.com/whisperchan/cursedboard is a messageboard with a text-terminal / curses interface, accessible via SSH. Not sure how I feel about this one, because of the potential to track literally every keystroke you do when you're dialed in, as opposed to simple web-messageboards that only know the committed HTTP requests i.e. which pages were read and which messages were posted. Or maybe I just don't know how to use it securely.
meguca, I heard, was a messageboard where everything you'd type was immediately sent to the server and displayed (see the privacy concern? but of course you could type your message externally and paste it in, but that's cumbersome and defeats the purpose), which was supposed to foster faster posting of smaller messages, as a compromise between chatrooms' few phrases per post, and messageboards' few paragraphs per post.
Do let me know if I misrepresented anything. Especially let me know of any other interesting messageboard systems.