Europa Universalis IV is the kind of game I wish I could play. Not just to impress my friends, although that's certainly a consideration, but because it looks like a remarkably deep and satisfying strategic experience. Of course, that's also the problem: Too much depth, too much complexity, and not nearly enough time, patience, or intestinal fortitude to power through it. So instead, I will simply admire from afar as EUIV Project Lead Martin Anward spends a few minutes talking about the changes and additions that will be made in the upcoming Cossacks expansion.
The "central feature" of the expansion is the addition of "Estates," which are groupings of power, like the nobility or the church, that exist within the borders of your nation and add a new layer of internal politics. Estates will have varying degrees of loyalty and power depending on how they're treated, and powerful estates can confer numerous benefits to your world-beating efforts. But grant them too much power and they'll rebel against you, and attempt to seize control of your nation for themselves.
Cossacks also adds a new Diplomatic Feedback interface that allows precise control of your attitudes and relationships with AI allies, and Favors, which can be conferred and then called in when needed. "Horde unity" replaces "legitimacy" for Steppe nomads as a measure of their loyalty and willingness to fight on your behalf, and there's also a new mechanic for the Tengri religion, a syncretic faith that allows certain nations of East Asia to choose a secondary faith from any that are either in or bordering their country, and then treat all provinces and countries of that faith as if it were their own religion.
Things start to get really complicated after that, so instead of trying to summarize I will encourage you to watch the video, and note that if this is (or may be) your cup of tea, Europa Universalis IV is currently on sale for 75 percent off—that's $10/£9—in the Steam Autumn Sale. The Cossacks expansion comes out on December 1.