Crusader Kings II was released in 2012, and promptly became my Game Of The Year. And that was in its paltry vanilla state! In the years since, it's gotten so much new content and so many big expansions that it's practically become its own sequel. The latest is called Conclave, and unlike the expansions which added maps and factions, this messes with a bunch of relationship stuff under the hood, like a "New diplomatic system" and "Revised education system".
Which sounds boring, but then this trailer -- as CKII's clips always do -- manages to explain not just how it works, but how it will be hilarious.
Conclave will be out soon.