Rome wasn't built in a day, nor was Civilization 6, I suppose. The strategic colonize 'em up is going to spend a year rolling out new paid (and some free) updates. Firaxis Games say "we want to bring you more content—more Civilization—but in a way that keeps the game fun." Apparently that's coming in the form of the New Frontier Pass, which will get you access to the six DLC packs coming to Civ 6. They'll be released every other month over the next year, beginning on May 21st.
Firaxis say that over the course of the six DLC's they'll be adding "eight new civilizations, nine new leaders and a variety of new gameplay content, including six new game modes." Firaxis say in their developer update below that the Maya will make a return. Never-played Gran Columbia will be another addition.
They also mention an "apocalypse mode," new buildings, new units, and new tile improvements. There are more updates than that and Firaxis say "we think you'll pleasantly surprised by a couple of them."
The first DLC pack will be the Maya & Gran Colombia Pack coming later this month. Following it will be the Ethiopia Pack in July. The rest of the four packs are unannounced so far, though it sounds like some bits will require you also own Civ 6's Rise and Fall or Gathering Storm expansions.
Along with the six DLCs, Firaxis say "we’ll also be deploying six additional free updates for all Civilization VI players. These updates will include balance changes and free content, among other surprises."
Even before a truckload of additions, Civilization 6 is one of the best strategy games on PC.
You can get the rest of the details about the New Frontier Pass on Firaxis's website. The Maya and Gran Columbia Pack releases on May 21st.