I'm not the most avid enjoyer of either of these franchises [just a personal preference, I find civ to be too "board game" like, when I prefer more simulation like games], but I was trying to think of why the new system [civ switching] felt off to me. Maybe this was obvious to other people but I finally realized what bugged me about it.
It's just too rigid. You're always switching from Rome into spain or something like that. But the problem is that it doesn't feel like your civ is evolving, because it isn't. It's just changing into a different one.
Imo, the best way to make these systems is to not have a "civ" at all. Rather decide the characteristics of your civ. This could be as broad as "sailing culture" to replicate civs like the Polynesians and phonecia, or it could be as specific as to what writing system you use, or if you even have one. But instead of just being "Spain but slightly different" it actually feels like you're going on a journey and forging your own civilization through a story. This would be great if you could get some anthropologists to work on it, along with political economists.
In another example, maybe certain traits could be decided over a long period of time. I.e, being stable could give you a trait that promotes staying at peace and not expanding, but at the cost of making changes in government harder and harder the longer you are in that position [i.e, pre-1911 china].
Or they could be instigated by some event and become more ingrained if they aren't changed. For example, you could choose between forms of government justification. Perhaps you would have bread and circuses, which would make you really stable as long as you have a surplus of food and amenities, but unstable if you lacked them. Conversely a divine right of kings would make people more docile in general but requires an organized religion and you need some religous or legal justification for wars against people on the same continent [or something. Idea is WIP obviously]. The game should also force some amount of instability on you, but should also make that a good thing in some cases. If you have a government that's too stable, like mentioned above, then maybe you slow down tech and cultural advancement, or economic ones. Or at a certain point it's just impossible to keep your government if the modern economy is incongruent with your civ. [This shouldn't require a complex pop system or anything. Just as you advance through the tech tree your settlements will have a system of deciding economic and political power of classes(as in, economic decides who the main producers of society are and political decides what change can be enacted). So if x settlements have dominant proletariat economic power but dominant Bourgeois political power, then in times of instability there can be a revolution to replace the Bourgeois power with proletarian power. [Note:this should actually be a tiered system, or have a third thing called control I.e, peasants and serfs could be the dominant economic power but can't actually take political power without the help of another class like the Bourgeoisie or proletariat. So a settlement could have peasant economic power, Bourgeois political fervor, and land owner political control.]
Obviously this does lose a large chunk of the appeal of civ being more board game like and leading a civ with a leader who both give bonuses you need to play around to win. But I feel like both humankind and civ 7 need to go "all in" on the idea for it to work, rather than going half and half and pleasing no one.
[Note: Obviously all of the ideas here are half baked examples. This came to me right after i woke up from a nap. Also no I will not try developing it myself because I'm not an anthropologist and more importantly my coding skills are less than abysmal. I more just wanted to rant because trying to figure out my problem with both of these games was bugging me]
I mean some rigidity is perfectly fine, especially in a game like Civ where the whole purpose is that you get to play as Rome and conquer the modern world. But in order to "play as Rome" you need to the game mechanics to limit you as the Roman civilization to make sure that you are distinct, at least initially, from say China, or France, or the Aztec or whatever. A fully "custom civ" would be extremely out of place in a Civ game, not to mention would certainly be imbalanced.
Ironically, the first 10-20 turns is where the majority of civilizations in Civ games are the most equal: 1 settler, 1 warrior, found first city, send warrior to explore around. A lot of civs also stop feeling different (at least up to Civ5) when you're in the end game, as, once again, most civs are sporting the same units and same buildings.
Not only that, you already make a lot of customization to whatever civ you pick as you play along. RL Rome didn't build the Pyramids of Giza or the Angkor Wat, nor had Japan as a next door neighbor, nor adopted tengrism as state religion, nor implement communism as an ideology or form of government. Making a fully custom civ is simply allowing players to choose and customize the base, the starting details.
Balance is always a problem, so how (un)balanced a fully custom civ depends entirely on what other customization options can synergize with it.
While you are correct that by the end of the game each civ looks the same, and at the beginning of game the initial starting strategy is basically the same, a lot of the civ customization is the flavor. England for example has a distinct art style, they have english music, their flag, and even their unit art. In Civ4 they reach their peak power at an era appropriate time, (red coats and banks) whereas Rome get's their legions at the beginning of the Iron Age.