Civiliation ability concepts for Civ VI

With Civilization V apparently at the end of its life cycle and unlikely to receive any more major updates or expansions, it's time to start looking to the future of the franchise: Civilization VI. Civ V was successful enough to spawn several spin-offs: Beyond Earth, Civilization Revolution 2 on mobile devices, Civilization World, and even a Civilization MMO. So it's reasonable to assume that if work hasn't already started on Civlization VI, then it will begin soon.

One of the things that I most love about Civ V is that each civ has unique powers that give them their own playstyles and flavors. The expansions (especially Brave New World) showed a lot of creativity with some of the civilizations. I hope that these design philosophies continue, and that we'll see some even more interesting gameplay variations in the new civilizations of Civilization VI.

To that end, I have a few ideas and suggestions for designs and themes for some of the common civilizations that are likely to appear in Civilization VI. I'll provide at least a unique abilility and at least two unique units / buildings / improvements, but I may also provide additional or alternative unique suggestions in case Firaxis decides to include even more variety and specialization. Since Civ VI will likely be a whole new game on a whole new engine, I can't give specific examples of the mechanics of these ideas. Instead, I'll try to focus on more broad concepts and maybe include examples based on Civ V's mechanics and features if relevant.

* NOTE: this post is a work-in-progress, and will probably be revised as I come up with additional ideas or clarifications.