In a recent developer update video, Firaxis unveiled upcoming content for the Civilization VI: New Frontiers pass. Last month, they added the Maya and Gran Colombia civilizations and leaders, as well as adding a new "Apocalypse" game mode.
This month, they're only introducing one new civ, Ethiopia, but they're making up for the seeming shortfall of content with some new twists on previous leaders, as well as a whole new element for those who want Halloween to come early this year.
The first two "Persona" packs coming out with the Ethiopia update cover Teddy Roosevelt and Catherine DeMedici. These create alternate versions of the leaders, not simply visually, but also in the mechanics and bonuses they bring to their particular faction. Prior to New Frontiers, playing Teddy Roosevelt gave players access to a unique cavalry unit, Rough Riders.
Now, with the persona pack, that unit only becomes available if you pick the "Rough Rider" version of Roosevelt instead of the "default" version. Meanwhile, the alternate DeMedici might almost be considered the "fun" version, focusing on collecting as many copies of luxury resources as possible instead of meddling in other nations' affairs with espionage. The Persona packs are tied up with the New Frontiers pass.
For those players who haven't picked up New Frontiers yet, new ways to play are still being added. Secret Societies add a whole new slot in the Governor table, but without the need to assign them to any one city. Instead, governor promotions unlock new units and structures which replace existing versions with appropriately themed variants.
The Owls of Minerva focus on gold generation, adding a shiny new variation on the Bank structure. The Hermetic Order goes more for science (or magic, or the confluence of the two), utilizing a new Ley Lines resource to supercharge districts and Great People generation. Meanwhile, the Voidsingers provide an almost-obligatory Lovecraft reference with a replacement Monument and Cultist units who can reduce loyalty (and presumably sanity) of cities held by other civilizations. Finally, the Sanguine Pact lets you play with a vampire replacement for the Scout, as well as creating vampire castles that can let you warp units from place to place.
There are several other leaders and civilizations yet to be revealed in Civilization VI: New Frontiers, including the new Ethiopia leader, but this particular update might well be worth the price of admission.