OP is a wall of text so I will respond in kind
a: AI is useless at war, period.
This is just an issue with AI programming, not really much to be done about it, it'll get better with time and updates.
c: City Capture Penalties.
I don't specifically agree that capturing cities should be penalized more but expansion in general should. There is no real penalty for expansion, this also makes the game favor war mroe as of course it's much easier to expand by conquest than colonization. In Civ 3, you had corruption and waste, in civ 4, you had increasing city upkeep, in civ 5 you had global happiness which IMO penalized expansion far too much and led to much of the world being uninhabited into the late game. In civ 6 there is really nothing, your luxuries don't go as far but you will get more copies of them, and tech/civic costs don't scale so the rewards for expansion and conquest are huge. But that is the only way to keep up on Deity I guess.
d: Warmongering "Penalties".
Yeah everyone tends to hate me for stupid reasons anyway. Catherine constantly reminds me that I know nothing before I even have the civic to build spies. I don't even know why the AI is denouncing me half the time. But at the same time the penalty for even capturing 1 city is enough for everyone to hate me for the rest of the game.
This goes back to the war issues, the AI is just not that good right now. There is nothing we can do except wait for updates, and no way to improve without learning from experience. There is no way for it to be competitive at the very poor level of play without huge bonuses. I haven't played on Deity yet, I can imagine how unbalanced it is makes it not very fun. But balancing the game around Deity bonuses being fun would make it imbalanced for normal/casual players on Warlord or Prince and I'm sure there are a lot more of those. Once the AI improved the insane bonuses will make it almost impossible to play without doing everything perfectly, I get the impression that it is not and you are trying to play normal games on Deity, I'd much prefer it to be like Civ IV where Emperor is the highest you can play a normal game on and to win at Immortal/Deity you have to do everything perfectly and exploit game mechanics to even have a shot.
I do agree somewhat. Because the AI is so bad a small army can easily pull off a conquest. Building and maintaining such an army is very cheap and easy and building a large one isn't that hard. But this is only true in the early game, in the late game I feel like units are too expensive, compared to how cheap techs are by the time you build units in some of your cities they will be obsolete, I would like to see larger armies. Maybe make the + unit production policies scale by era so it would be +25% for ancient/classical and up to +100% for atomic/information units.
Districts and Wonders consume workable tiles; essentially reducing poductivity of larger populations.
Don't really have an opinion on the rest of this, I would like to see more housing options (lol) but I hate how Wonders consume tiles, if you have a capital with mountains/coast and build many wonders you will run into serious space issues making it worse than new cities with less wonders. You should at least still be able to work the land.
Haven't played on Deity but yeah, I'd imagine getting a production boost is overpowered, the boost to tech and building districts/builders should be enough, this should probably be removed.
2: Imposed Scarcity of Land.
I really don't agree with this, I'd rather see it this way with more early game interaction and colonization later on in the game, I usually play on Large and haven't tried below Standard but it seems there is enough map that it takes most of the game to settle, everything gets settled and a lot of the newer land is not fully developed. Compared to Civ V especially which did not do a good job of this I think it is good.
3: Pacing and Great People.
a: The early game and mid game ends too early because the AI advances too quickly.
I haven't played on Deity but I can definitely see this being a problem. Yes beelining into the atomic era while still using caravels and muskets is stupid. It should calculate average tech level or something or at least make you reach the new era in both tech and civics to qualify.
b: Some great people are just plain overpowered and their abilities favour very different victory conditions that their type would suggest. They're all great people; why are some greater than others?
Eh, part of this is flavor, I don't really agree with the specific issues you complain about, part of the deal of putting very highly powerful boosts for one victory inside another victory's progression is to enable you to compete at something you haven't been focusing on all along, you shouldn't be able to just stack everything when focusing on one victory like getting all the tourism boosts while doing culture. And maybe if you don't have the production late game but plenty of money that extra tourism boost can be what you need to switch from science to culture victory.
But yeah there is definitely a pretty huge balance issue, like +100% space race production vs just 1 or 2 late game tech boosts, theres just no comparison.
c: Great People points. (GPP)
Huh, I've hardly ever built any projects and just get great people from points or money/faith, although again I haven't played on Deity, interesting that it is that huge an issue.
I kind of agree with this, late game production times get extremely slow for districts units and wonders, meanwhile tech and civics are actually speeding up, I'd like to see production speed up as well. Best way I can see to do this would be adding some % production boost to factories and power plants instead of just flat cogs, perhaps have both with the cogs spreading to nearby cities and the % boost only to the city that builds it.
b: It is impossible (You can't compete for one) to found a Religion on Deity if your opponents favor Religion (Unless you're China or Arabia).
I really don't see this as being a problem at all. In fact I see it as being highly desirable. Deity is supposed to be really hard. You're not supposed to be able to just play a normal game as any civ with any strategy and have it work. If you really want a religion on Deity, play as China or Arabia. If getting a religion is so easy that it's doable on the hardest difficulty without even picking an optimal civ then it is too easy, period.
Founding a religion is balanced around Warlord/Prince/King level of difficulty that most people play on. There is no way to balance it to be mildly challenging on Deity without making it completely broken at this level, and there is absolutely no reason to.
Yeah I agree with this. Wonders are very very expensive and some are just not that good. It's always worth building the Forbidden Palace if you can but then some are just worthless,
Yeah I don't fully understand how theming works but couldn't theme a single art museum in the late game going for a culture win and ended up theming an archaeology museum in a conquest game by accident. This adds to those cultural wonders not being that good. The whole great works system is kind of confusing and the problem is it just isn't balanced to work too well. It should be pretty easy to have themed museums with great works from different eras if youve had a few great artists, it really isnt.
8: Diplomacy
a: Most AI Agendas are beyond the player's control and don't even make sense.
I completely agree with this, it's a good idea but the implementation is awful, I'm constantly getting denounced or insulted by random leaders with no explanation over things completely outside my control. Catherine in particular constantly complains that I don't have enough spies and within a few dozen turns would both insult me for being broke and compliment me for being rich when I was running about +50 gold the whole time. Warmonger penalties are just absolutely ********, I get hated by every civ, all game because I took 1 city in a war someone else started when I've been at peace the entire rest of the time.
But again, this is largely AI optimization issues that will be improved in time.
b: Diplomatic Penalties are insignificant.
Unfortunate but necessary with how broken diplomacy is in general.
c: There should be more options available, such as Ultimatums.
Civ has never had that kind of detailed diplomacy but yeah, there is already Casus Belli and it is EXTREMELY annoying when someone settles between your cities on what is obviously your clay and there is no option but full blown formal war and every civ on the map hates you for the rest of the game. Take a page out of EU, you should be able to proactively warn opponents not to settle near you and if they do, get a Casus Belli to take that city and only that city with greatly reduced warmonger penalties. But of course that would require warmonger penalties to work.
9: City States
a: City states are still too vulnerable. The AI is way too aggressive against them.There should be penalties for Conquering City States, like loss of Envoys from other city states and even the permanent war against city states for conquering too many.
Yeah, getting -1 envoy with all city states for conquering one would be fair. It would also be neat if you could somehow sack/ransom them instead of just full blown conquest.
c: City State Insurgencies.
Seems to be pretty straightforward, when a city has too much negative amenities for too long it spawns rebels, city states very rarely build entertainment districts and have at most 2 or 3 luxuries so they can support a max of like 6-8 pop and end up with 12+, constant rebellion is inevitable, gives em something to do with all the units they build anyway.
Most elegant fix would be to somehow have city states trade resources with each other or with civs to make that up, would prob be hard to implement. Alternative fix could be to have them share luxuries with their suzerain, either integrate them into however the hell the amenity distribution system works, or just have them get a copy of all the luxuries from their suzerain. Then late game city states need to be aligned with a major power or are highly unstable and prone to rebels, that sounds like the ideal solution. Cheap and dirty fix would be just add amenities to city states in later eras but something needs to be done.
d: Reward for being the first to meet a City state.
Nah, needs to have this, it's random but so much about the start of a game is random and affects the gameplay so much, it's fine.
You have some valid points and I tried to give more feedback than just stop complaining about Deity being unbalanced but...stop complaining about Deity being unbalanced. The game is balanced to a more casual difficulty level, this isn't going to change and there's no way to fix it. Deity isn't supposed to be balanced, it's supposed to be prohibitively, disgustingly hard. As it is the AI is such a joke in key ways that extreme boosts are the only way to make it competitive. But as I've said there is no way to fix this but wait for improvements in the AI. I feel like if you were playing the game in a year or two after the first expansion, all your issues about Deity being hard would still be valid, but you'd actually have a challenge playing on Emperor/Immortal where you can pursue a more peaceful route rather than exploit every possible advantage to be able to win.
sorry for the tl;dr can't sleep