Late to the topic, but just trowing my two cents in the main topics... I'd skip loyalty, however as I've not read all the discusion - and it is probably one of these "good only if improved" topics we'll need to extend a lot about. On other topics:
City States: Agree basically with
@AntSou initial point about city states, but
@TheMarshmallowBear 's comments about envoys are on the point: I would like to add here as maybe a minor improvement (that could be also applied to eurekas/inspirations) that maybe some features have made too simple for them to be enjoyable: this is the case of missions for city states - there is just one mission at a time, which is not specially cumbersome to accomplish, and then nothing. ¿Why do not have an higher number of missions of this type, running on parallel, but with less reward?. And then, if you want an high-reward mission, make it more complicated, history-driven and maybe multi-layered (CivBE three-step missions come to mind).
*I take the opportunity here to introduce the point about
eurekas/inspirations: I would also think the game would be much more dynamic and interesting if, instead of having just a single eureka/inspiration providing 40% value, each tech/civic would have around 4 eurekas/inspirations, maybe a bit easier to accomplish, providing 10% value each. This would make more difficult to just play for the eurekas (as there wouldbe much more work to do), but still will include the map/gameplay tech selecting factor, as determinate map locations/way to play will easily trigger all the eurekas, and others none (and you will have 1, 2 and 3 eureka locations in the middle...). I think this kind of approach (altough probably implying more work to code) would contribute to the "less binary" need that I read in some posts.
Districts: Agree it is a must stay, and (opposite
@Alexander's Hetaroi in this case) I don't mind city spread: you need to switch thinking of cities as cities, and more as "provinces", and this works both in ancient times and modern times.
-> minor and "fun-factor" possibility - having the option to "name" districts might be immersive to this way of thinking (e.g. Athens' dock could be named Pireus, Rome's dock Ostia, Madrid's campus Alcalá and London's Oxford, or Los Angeles theather square Hollywood). This naming could be automatic (but editable - sort of how it works with cities now) and hidden if you don't toggle it on, so it does not interfere with quicker games.
Regarding buildings, I think an intersting flexibility compromise would be to have a space in city center to build lvl1 distric buildings: this way you could have a market, or a workshop, without need for the full district. To not mess too much graphically, this slot could be limited to 1 district lvl1 building at once. And this building would be moved to the district once placed, to make space in the city center for a different one.
I agree as well adjacencies might be revamped and made more dinamic, having more options and tying some of them maybe to (alternate) buildings (as above, make the system less binary). And I'd like to see as well alternate buildings depending on adjacent districts (e.g. Science Lab might be substituted by an Economics Faculty providing gold bonuses + science IF a CH is adjacent to a campus, or to an Engineering School providing production + science bonuses if you have an IZ adjacent to the campus).
In general, regarding comments about
victory types, i have mixed feelings about how they were implemented in Civ 6.
Religious victory, in special, I'm in favor of just integrating in cultural victory (as an early way to achieve it or a contributor in the long term). Religion may have as well long-term impacts in Diplomatic victory (as a source of favour/diplomacy points). Science (Space) victory is the one that irks me the most as I'm ok with the rationale for it being a way to win, but does not seem a reason to lose at... (well, a civ just migrated to other planet... why do you lose in this planet?

).
On diplomatic and work congress I'd comment
@Eagle Pursuit : I like the ideas on how WC is working on Civ6, but I cannot like how it is implemented: it seems quite rushed, with options coming out the blue and not being integrated enough with other system. Of course, I don't think Civ V option to having just the leader define the agenda is the best one.
(I think I commented some time ago on more complicate options: having you governors become delegates of several WC "comissions" and your strenght in each of the "comissions" areas (science, culture, military...) allowing you to influence the agenda - this could be an innteresting way to handle the WC)
And.. well i think this is long enough