Resurrecting this thread. I see that CA interviewed the 1212 AD mod team regarding their work a few days ago, apparently (at least in part) because it received a major update just last week.
https://www.totalwar.com/blog/an-interview-with-the-medieval-kingdoms-1212-ad-team/Jackie Fish posted a video highlighting the changes, improvements, and new features:
To start off with, it sounds like the single most notable feature added is that the HRE electors have now been implemented -- get yourself elected Emperor! On top of that, most factions (though not quite all) now have their full army rosters, including their mid- and late-game units.
You can also now trade regions, so as to avoid that unsightly border-gore.
Speaking of regions, it's also easier to play "tall" now, as they each have ten building slots -- you just have to be patient enough to "grow" your cities (recruiting armies drains population from a region).
Still to be implemented: Papal elections/College of Cardinals, mercenaries, naval battles, republic mechanics, and custom/faction-specific buildings. it sounds like mercs are the next major feature the mod team is working on, and modeling work is starting to focus on the custom buildings.
I may have to jump back into this one, dang it.