Dynamic Civs is something I've thought about myself, but it's not that easy to come up with a well-designed system. We've seen games such as Millennia and Humankind do it - with mixed effects. I haven't played Humankind yet but it appears their Civs are a Smörgåsbord of different cultures fused together (folks that played HK pls confirm or deny this) and that seems to work just fine? Millennia's Civs are bland and without personality, which makes sense as the game is still in Early Access.
But is this what Civ wants to be?
Civ has definitely grown throughout the years as a game that's been trying to improve its historical accuracy in how it portrays its Civilizations. We've seen this massive growth for ourselves in Civ 6. The big blob Civs have largely left, with a few exceptions (India), and I think their system still works the best. Two abilities, two uniques, and each leader represents a certain time period in history corresponding to their reign and what the Civ was doing at the time.
Even compared to the Civs that launched at vanilla and those that were released with NFP we saw some differences - With the exception of Babylon, the NFP Civs were balanced between good game mechanics and representative abilities. Vietnam, Gaul and Portugal are among the best designed Civs in the game, which is impressive given how late they joined the line-up. Compare that to vanilla civs such as Greece and Germany who are effective but feel generic, or Sumer which is a full-blown in-game joke, you can see a lot of growth there.
Then there's the split of the Greeks, which is a bit controversial, but generally well done. I DISLIKED Macedon when it was released because it came before the first XPac - at that time we already had three hellenic leaders (Pericles, Gorgo and Cleopatra), and Macedon being a separate civ from the Greeks felt like a janky decision at the time. Since then though, I think splitting Macedon off was the correct decision, and believe that Pericles and Gorgo should have just led Achaea and Lacedaemon proper, rather than Greece which was never a unified empire at that point of the timeline.
However, the one constant draw to Civilization compared to Humankind and Millennia is that you ALWAYS felt like you were playing the Civilization you chose at the start of the game. Civ6's Civilization design nailed the personality - Mongolia is Mongolia; Inca is Inca. Rome is Rome. They're doing the things you want them to be doing.
So, what system should Civ7 use? One similar to the one they used in Civ6 but more organised, and that build up starts when they first design their Civs. The distinct ready made identity of its Civs is its biggest draw and should be retained.
Civilization 6 worked on two levels - the level of the Civilization, and the level of the Leader. Each level had an ability and the leader could only be played with their assigned Civ (or Civs).
I believe that Civ7 should add a third, intermediary level between Civ and Leader. This would allow for better designed Civilizations and more accurate historical representation (not to mention it would create an organized system modders can use to slot in their own playable Civs and leaders):
As you can see: The Civilization level would still exist, but be an overarching factor for empires that have existed at separate points in the timeline. I've used Rome and Byzantium in this example because the latter saw itself as a direct continuation of the former, and retained several elements. These overlapping elements would be covered by the Civilization level - values, ideas, units, etc that would be shared by all Playables. Examples of this would include India's Stepwells and Greece's Hoplites.
The second level I would call either the Dynasty level or the Empire Level - We already have Civs that exist at this specific level - the Byzantines, Macedonians, Ottomans, Gauls and English all fall within this group. I would say that EVERY Civ needs to be in this group going forward. Deblobbing Civs such as India, Celts, Vikings and Greece will necessitate the existance of such a group.
Uniques at this level would represent their empire's zeitgeist. This would be the Jannissaries for the Ottomans, the Redcoat for Victoria's England (which really should have been called
Britain), Byzantium's Hippodrome and the Qin Dynasty's Great Wall.
Furthermore, it's the second level that should determine the icon, jersey colours and city lists. It makes no sense for Eleanor of Aquitance to settle Industrial Era British cities, no more than it would for a Soviet alternative leader for Russia to be using Russia's eagle banner and Lavra district.
Other examples of Civ-into-Empire splits would include
Germans: Holy Roman Empire, Germany Proper, Prussians, Saxons, Bavarians, etc.
French: French proper, Franks, Burgundians, Occitans, etc.
India: India Proper, Mauryans, Mughals, Dravidians, etc
Chinese: Han, Tang, Song, Ming, etc.
Celts: Gauls, Bretons, Irish, Welsh, Scots, etc
English: Mercia, England, Great Britain, etc.
Persians: Iran proper, Achaemenids, Sassanians, Parthians, Safavids, etc.
Arabians: Abbasids, Ayyubids, Yemeni, Omani, etc.
Turks: Seljuks, Ottomans, Türkiye, (Timurids?), etc.
Greeks: Athenians, Spartans, Macedonians, Myceneans, etc.
Egyptians: Egypt Proper, Hyksos, Ptolemies.
Algonquians: Iroquois, Cree
Indonesia: Malaya, Majapahit, Sundanese, etc.
Bantus: Zulu, Xhosa, Zimbabweans, etc.
Nigerians: Yoruba, Beninese, Hausa, etc.
Not every Civ
needs to be split up this way, but if you do your research as a designer, surely you can come up with three abilities and three uniques for stand-alone isolates such as Georgia and Nubia?
Finally, we have the leader level, which is an essential part of Civ. Leaders are the face of their empire, representing its values and playstyle. Leader abilities I think are optional, but can be included for flavour (and again, modding). The more important aspect for me is their personality, strategy and agenda
Personality is how they interact with other players and respond to behaviour. Are they belligerent, or pacifistic? Are they likely to honour agreemens or stab you in the back? Civ 6 tried to use agenda's and this turned out to be a poor choice. Civ6's AI is poor, because it is forced to piggyback off its agenda too much and too many of the modifiers are hardcoded;
Hard-coded modifiers are important though, which is how Agenda's should be used going forward - clear parametres of how the leader would play, to truly make their personality shine. Finally, the strategy a leader has (functioning similar to the Hidden Agenda), would determine their strategy to win the game, and their main focii.
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And now I'm going to address some of the other points OP made, because they're good. However, there is an issue with them that i'll address at the end of it.
Splitting civs: I think it's viable for Civilizations to split, if the system allows for it. If Rome is in the game, and their have extreme internal discord, why not make Byzantium (if not in the game) a spawnable Civ? Does it even have to be Byzantium? I think that's perfectly doable. The Civ that splits off retains the upper level abilities, and has the rest randomized. You can make that a basegame feature or a game mode.
Ascending civs: I feel like the same argument can be made for powerful City States - if a City State grows powerful in its own right, it could potentially ascend to become a major civilization (similar to how in the Barbarians game mode, clans can
ascend to become City States)
Assimilating Unques: If one Civ conquers another Civilization, they should at least retain the Unique improvements of the Civ they've conquered. It makes no sense for all Sphinxes to suddenly
vanish each time I capture an Egyptian city as Macedon. What is funny though is that Macedon's ability, Hellenistic Fusion actually references exactly that? How Macedon took elements of Egyptian culture and merged it with their own to create stability in their newly conquered realm? This not being in the game feels like an oversight (and is one of the few things I specifically have a mod for).
Merging: This can be added by allowing Civs to surrender to each other in diplomacy. However, if the AI is poor at making decisions, they could potentially surrender from a non-losing position, and that could result in the player they made the surrender to, to snowball out of control. I've seen this in MoO2 many times and each time it happens to an AI and not you - :sigh:.
Now here's the issue with some of the other points -
they complicate the game, and Civ 6 was already a fairly complex 4X game (not NEARLY as much as EU4 is, but like... it's EU4). For the vanilla game, I think you should go for straight-forward systems that you build upon later, and that the computer player can use properly. A lot of these feel like they should either be game modes or XPac mechanics.