I'm playing Crusader Kings II: The Old Gods, a role-playing grand strategy game by Paradox Interactive.
This expansion adds a new start date (867) and the ability to play as pagans. Pagan nations have a number of new rules: raids, provincial conquest and invasions, forced gavelkind succession, and special events like human sacrifice. All nations now can also spawn adventurers, existing characters that go after specific targets like a claim on a province. Everyone also gets an overhauled technology system: points are generated according to nobility character stats and buildings placed in your provinces. Lastly, rebellions are now given a leader with an objective. Overall, the new features of The Old Gods add just enough variety to the existing game to justify the $15 investment if you plan to focus on the pagan nations and the earlier start date.