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The latest Civilization 6: Rise and Fall trailer takes a break from introducing new leaders and nations to provide an overview of the new systems that will be ushered in by the expansion, including Great Ages, Loyalty, Governors, Enhanced Alliances, Emergencies, and new Wonders and Units.
The primary addition, Golden and Dark Ages, are temporary changes to a civilization that last for an era. Both can dramatically alter the state of the game and will force players to adapt their strategies accordingly, but while Golden Ages are obviously preferable, Dark Ages have upsides too: Golden Ages come more easily when emerging from a Dark Age, and they also enable Heroic Ages, which grant three Dedication bonuses instead of just the single one that comes with a Golden Age.
Changes to alliances also promise to make things more interesting, by making the alliances themselves more meaningful. Instead of merely ensuring that other civs (hopefully) won't drop the hammer on you while you're preparing to do the same to them, Rise and Fall will enable different types of alliances—Research, Military, Economic, Cultural, or Religious—that will provide better bonuses the longer they're maintained.
Emergencies are similarly designed to encourage diplomacy and cooperation. When one happens—for instance, someone nukes a city—the other players have the option of targeting the aggressor with a joint Emergency action, which will give them a specific objective to complete within a limited amount of time. Completing the objective can confer permanent bonuses to all who take part, but failing to get it done will grant a benefit to the intended target instead. And civilizations don't have to be allied to take part in an Emergency, so doing something to trigger one could have the knock-on effect of bringing together forces that were previously unrelated, with their attention turned to you.
Civilization 6: Rise and Fall comes out on February 8. If you don't already have the base game, you can pick it up along with a couple of DLC releases for a really good deal—$12, instead of the regular $60 for Civ 6 by itself—in the current Humble Monthly Bundle.