The biggest issue I have with civ switching is tied moreso to the 3 era system. As some who really likes to "write" my own historical narrative in each game, it will take a lot to have me get past these forced situations where you need to switch civs to get through a crisis. But the forced narrative is a wholly different rabbithole. The other issue, as has been beat to death here is that the 3 era system of locking civs means that each game is going to be painfully similar since you have the same civs and besides the leader bonuses, they basically will play the same way.
However, civs do change and I like that they embraced that idea. However, I think they're going about it in a terrible way to try and preserve the idea of a "civ" per age. It's mindboggling that this is how they approached it and this does mesh with the aforementioned age issue. Why do my ancient and explration era bonuses of the civ go away? That is just killing me internally as I think about it.
Getting the opportunity to add bonuses or government types or whatever to your civ in a new age is totally fine and in my world, I would have much rather played as the Egyptians who over time gained the traits of the Mongolians and the Americans. As such, I would have rather built my own civ over time instead of switch between them. While I would always be "Egypt" as to reference my starting civilization, perhaps they could have a system that when you unlock the bonuses of, say, Mongolia, you could slot in a card and have those bonuses get boosted somehow. So if it's the exploration era, you could either double the Egyptian monument-building bonuses for a while and then at any point (i.e. when you've finished your wonder) switching to doubling the Mongolian bonuses towards calvary. Your old bonuses wouldn't evaporate; they just wouldn't be doubled which would be a strategic decision players would make. This would get even more ridiculously fun in the modern era with three different options! Something like this wouldn't erase or not include the bonuses from previous civs and eras; it would just give you the power of what to highlight. Like, if the Mongolian war bonuses are really helpful in the modern age you should have the ability to keep them on full blast even after your forced switch (Or in my case, a forced "add").
The other MAJOR thing that this would help with is the fact that holy hell; there are SO many bonuses for each civ in Civ VII. I find it wholly ovewhelming to think about so many bonuses that will switch multiple times! I see it as they added all these tiny bonuses so that each civ felt more complete in the absensce of a specific leader as we've seen in the past. But...I find that system even more irritating. I also find it extremely annoying that you have to be playing the game with the mindset of "hmm im gonna lose my river bonus next era so I guess I should not settle any more rivers". Why couldn't that bonus remain and you just get an option to accentuate different parts of your 2 or 3-part kit? They could even (naturally) make the exploration and modern era civs more powerful (As they should be) to encourage people to double those bonuses instead of just sticking to same ancient era bonuses. It almost makes too much sense in my eyes. If using an ancient era bonus in the modern era is OP or unbalanced that could be an awesome possibility to discover that could add flavor to the late game. You'd still be America and have their bonuses (Hopefully just fewer) but would be doubling your early game bonuses to accomplish a task. And tbh if it is too OP or game centralizing they could patch it out which they have the potential to do. Either way, since your bonuses from the first civ do not go away each civ could just have fewer of them which would make it less overwhelming for anyone who doesn't play the game 24/7 or has time to dig into the weeds of so many.
The one issue I could still see is that it does make the identity of the civ very dependent on the ancient era civ/culture. But if I'm being honest...that's not a bad thing. Yeah this system (Since it retains previous civ bonuses) would make players think of each civ as a their ancient era counterpart the most. However they could just include a lot more ancient era civs to represent most world cultures and again, incentivize the doubling of later era cultures' bonuses. That way if you really, really wanted to be French, you could double those bonuses to make it feel like you're really adopting this new playstyle in the modern age to you, while outwordly still being perceived in the same way. Plus, even though I'm making an amalgamation of a civ over the game, it's still held in place with the "rock" of my original choice which helps with roleplaying. Like, yeah I'm fighting the Mississippians that added some Spainish attributes but they're still the Mississipians. And if the issue was the name of the civ...I mean they could either double down and name the empire after the leader itself, or, allow the player to literally name the empire. I wouldn't mind having Napoleon lead the Egyptian-Mongolian-American "Napoleonic Empire" or Himiko lead the Mauryan-Norman-Bugandian "Himikian Empire". And for gods sake they should have just had each player retain the same colors bc I really don't want to play a game where civs are constantly changing from blue to black to green throughout the game...nevermind how that'll make things difficult for colorblind people.
Any or some of these solutions would have made civ switching work in a way that didn't divide up the game so harshly...but it seems they really wanted to make it a history simulator and force the same 10 civs to be in every game at the same time. Why couldn't a Greek-Ming civ exist alongside a Mauryan-Spanish? Why does it HAVE to be held to the same era? It just smells of uninspired development and wanting to take the easy road of "making three games" instead of an actually involved civilization game...and I don't like that.
But the system as is, alongside the ridiculous paywall increases ($60 for extra content...at launch!?!) makes Civ VII somethig I will have to explore 3-4 years from now. Hopefully by then they make some significant changes that don't downplay your earlier era achievements or let you not use them for some reason...although I doubt it. Knowing FXS they'll focus on making more civs and leaders which is IMO pointless if they aren't going to take in advice and make big changes. I know FXS can change core gameplay (See Civ V to BNW) so I hope that however the game plays, they are willing to change their products' systems. And god knows I want to be wrong and see this as the greatest civ game to ever come out...but I remain unconvinced by everything I've seen so far.