I have recently been unable to participate in the GotMs due to sudden lack-of-free-time syndrome, but I wanted to say thank you Thorak for your contributions! I am really excited to see more games being posted
Now regarding feedback, I think that the three topics (difficulty, modes and victory conditions) are actually pretty tied together in a lot of ways, but I will try to address them separately nonetheless.
1. Difficulty
I prefer Deity games. This has nothing to do with how the AI does later on, but mostly because on lower difficulties it becomes pretty trivial to bum-rush the AI in the Ancient Era, to the point where playing in any other way puts you in a disadvantage. On Deity (8), perhaps even on Immortal (7), the combat bonuses are large enough to make early warring more situational, and as such it increases the decision depth in the first era. Unfortunately, this also means that newer players might be too scared to try a GotM when they see 'Deity' level in the description. Conversely, think that even lower-level players are capable of beating Deity AI, they just need a bit of help from the seed and game settings.
2. Game modes. I can segregate them into 4 groups:
i) Never:
-Zombies. It's a horrible, horrible game mode, that invalidates the entire experience of playing a Civ game. AI can't beat it, and any longer games just become tedious micromanagement hell which drags on forever. No zombies, please!
ii) Unadvised:
-Dramatic Ages and
Apocalypse. These two are not so bad, but I also don't think we should include them. I have yet to see anyone who enjoys them often - the first one shapes the gameplay into a weird rubber-band loop, where the better you do, the harder you get punished. The gameplay is shifted towards only caring about managing era scores, as the AI will colapse eventually to a dark age. Unfun, and also, not what Civ is about at all. Apocalypse is better, but I just think that it doesn't add much to the game, it just clutters the gameplay with 'Appease the Gods' emergencies.
iii) Game-breaking:
-Secret Societies and
Heroes and Legends. The first one offers a huge boost to the human player, while the second one makes it almost impossible to lose. I think this is a good place to refer to my notion regarding managing game difficulty - adding Secret Societies could be a good way of making a Deity game much easier for newer players. For the second mode, Heroes and Legends are incredibly busted and if they are ever enabled, they should be accompanied with a brutal starting location and a weak civ.
iv) Good stuff, the modes which can be used along certain other settings to make the gameplay more enjoyable:
-Tech and Civic Shuffle is interesting, and usually makes for a fun game. I wouldn't enable them often, as the core gameplay of Civ6 is pretty well balanced around the trees themselves, but they can make a Science and Culture games more enjoyable. Probably not a good mode for Domination games, as it makes it difficult (or rather too random) to plan your strategy.
-Monopolies are a game mode which is a nice addition in Science and Domination games, they make the game a little bit easier, but in a minor way. For me, they can be pretty much enabled always. An important exception are Culture games, where the Monopolies have to be disabled, otherwise the gameplay will be railroaded into grabbing lux resources, as anything else is suboptimal.
-Barbarian Clans can easily clutter the game and produce way too many City States for usual games, but they can be turned on on any Archipelago type maps. For those, it allows the players to actually control the barbarians and the game does not turn into 'Zombies on sea' mode, which can happen with regular barbarians on water maps.
3. Victory types
Science is the most fun way to play the game, as it promotes a most balanced playstyle of empire-building. All yields are useful, and so the decision space is at its peak from the very start.
Domination can be very fun on specific starts, as it can lead to something I call 'Science-Domination' game. It usually culminates in warfare which makes use of a wide spectrum of units from all ages, as long as there are powerful civs which are not located on the same continent. In other words, Domination games are best if there is a distant land with an AI that can't be reached before Cartography. Pursuing the science tree in order to gain a military advantage is way more fun than rushing Horsemen or Men at Arms and rolling over your neighbours

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Culture is the hardest, as the AI can really run away with it if you don't get a good start. The only thing I dislike about Culture is that whenever I see that the AI are easy to find early on (for example on Pangea) then I know that Reliquaries are probably the way to go, and I really really hate Reliquaries-based cultrual victory. It's more of a speedrunning gimmick than playing Civ6.
Diplo/Religious/Score - I really don't care about them. I don't think they are in any way engaging and never opt to play for them - stringing them along with cultural and domination victories as secondary conditions is a good idea.
Finally, I am all for additional challanges, which don't have to be enforced by the game itself. I absolutely loved the 'peaceful' challange for one of the Christmas games last year, I think more stuff like that would be great

. Adding such optional modifiers could be an another interesting to make agame more difficult for the most experienced players, while still allowing more casual players to participate (as the modifier is optional). Some other wild ideas: what about Domination game where you have to burn all the conquered cities, a Science game where gold trading is forbidden, a Culture game with no wonders? Maybe this year Christmas game could be a nice experiment
