If we ever get vassalage back (in a third expansion or in Civ7), this would actually be a great ability to include, because it's very different from the Greek or Phoenician city-states, which were sometimes at war and sometimes in league but rarely in direct vassal-sovereign relationship, whereas this happened repeatedly in Mayan history. Just not in the form of the return of Civ5's city-state buying abilities. Those were annoying...
Hmmm... Maybe gaining the Suzerain bonus of a City State after conquering that City State? Or "Can't annex city states, allying a city state blocks all other civs from sending envoys to that city state." I like either idea and it would suit the Mayans and their permanent league wars. Make it happen, Firaxis.
There seems to be an unwritten rule at Firaxis: European civs must be bland.
No excuses for Germany and England imo, which have a myriad of fun abilities (and I think Germany especially is well-designed in Civ 6 at least. The less said about England the better). Like I said,
France is the hardest to find good abilities for because the things they are the most notable for (Hundred Year's War, Maginault Line, Grand Tour, the City of Light, early use of Cannons, Châteaux) transition into bonuses that are either boring
or generic or
both boring and generic. Their real life civilization was built on early war, early-to-mid engineering and mid-to-late game patronage of the arts - this is sort of how they work in Civ 6, but that always felt accidental to me (CdM became a good warmonger after they boosted DipVis, but that was mostly done to make the Mongols stronger, rather than CdM)
Okay, that makes more sense. Also I suspect a lot of the scholarship is in Spanish.
Precisely. Most of it hasn't been translated into English yet. I can read Spanish, but my active knowledge isn't good enough look up information I need (such as indigenous names for key concepts such as Pyramids and their bloodletting rites) AND THAT FRUSTRATES ME.

(I could of course learn Spanish, but I hated my high school Spanish teacher, so nope.)
The discovery of Hittite was a huge milestone for a whole slew of disciplines, especially Indo-European linguistics but also Assyriology, Semitic studies (the Amarna letters!), and several others.
The Hittites mostly benefit from their location, actually. The Turks are very proud of their ancestry (um, ignore that the Hittites were Hurrians and the Turks are of Turkic origin) and most Turkish tourist websites have detailed, but succinct summaries on Hittite culture, which makes learning about them a blast. Here's a
link to one such websites. (and I freaking hope they get added in Civ 7 as well. Not Civ 6 because most of their abilities have already been taken by other Civs (the reviled "+2 movement after a declaration of war" is a PERFECT ability for them), though Telipinu and Puduhepa are very interesting characters that deserve more attention than they've been getting)