Civilization VI was rightly lauded as a return to form for Firaxis following the unfortunate diversion that was Beyond Earth. But, as you'd expect from the first iteration of a complex 4X strategy game, things haven't been perfect. An update last month brought DirectX 12 support and a considerable interface upgrade, along with the standard balancing and AI improvements. This month, the game's developers are adding more features, refinements and, for a price, a new civilization to toy with.
For full notes on what's new, you can head to Firaxis' website. There's no headline item per se, but the addition of an "Alert" action that lets you sleep units until they see an enemy, and killing the bug that had Great Admirals randomly spawning on top of wonders (making them functionally useless). Rest assured there are plenty of balancing, AI tuning, aesthetic and bug fixes that should add some polish to the game.