In Civ 6, you can win a game perfectly fine on 5-7 cities, each with a governor, the right policies, et cetera. Meanwhile, it was virtually impossible to expand in Civ 5 without crippling your happiness and cultural progression - and to a lesser degree your scientific progression.
I am quite sure you can also win with one city in civ 6. I am also quite sure victory in civ5 was not automatically locked after acquiring Xth city.
Having one less social policy/ideology tenet was not truely so gamebreaking.
Anyway the direction of arguments leans heavily towards the balance discussion. I don't want to follow that. Existence of optimal strategies is certain and does not mean other do not exist.
To not leave you without response I am gonna link T-hawk's blog, quite detailed explanation, wide variety of gameplay and documented history of civ 5 through expansions:
http://www.dos486.com/civ5/bnw16/ (at the bottom there is a table of contents)
I'll do you one better: I've attempted tall games in both Civ 5 and Civ 6, and I enjoyed them more in Civ 6. Let alone the difference between wide games.
Oh, I completely agree tall games are more interesting in civ 6. In general auto-turn processing have a better feel due to amount of checkboxes (for example eurekas).
Wide games on the other hand... Though again, opinions, just because you don't enjoy something it does not mean it is fundamentally broken.
If this makes sense, in Civ 6 I feel like there are incentives to not expand, while in Civ 5 there are punishments against expanding. And I'd much rather be led by incentives than punishments.
You know in some race cars games, there is a system that every continuous player is moving a bit faster (the last one is the fastest). The point is that speed is relative. So do feelings. And so do rewards/penalties.
At one point Settler/Builder/Districts costs PENALTIES may hit enough for player to feel that expanding is not worthy. For the conquest, I have no idea, I heard rumours of grievances, war weariness, emergencies, diplo favour?
Older players may also suffer from carpal tunnel syndrome penalty.
All that is of course ignoring that the entire concept of tall versus wide did not exist until Firaxis included it in their Civ 5 design philosophy.
As for the unhappiness... the system is not complicated. That doesn't mean it isn't the single most annoying game mechanic I've ever encountered. It punishes you for building a new city, it punishes you for conquering a city, it punishes you for growing your cities, it punishes you for any action that's supposed to grow your empire, and it does so by cripping your empire. In Civ 6 (which technically has roughly the same calculations for happiness!), if you don't meet happiness demands, individual cities drop into negative happiness and receive sizable penalties to their production and growth. In Civ 5, if your empire as a whole lacks even one point of happiness, you get... what was it again? -75% food in all cities, or something like that? And if I remember correctly, that wasn't all, but it's been six years since I played the game.
-75% Food penalty sounds very scary. Of course like most Food modifiers it is applied to surplus food (after the cost of citizens was applied) and automatic management instantly moves citizens to optimize other yields. It is also extremely logical, because extra Citizens would generate more unhappiness. It applies for both civ5 unhappiness, civ6 housing and civ6 unhappiness.
As you noticed civ6 also have very similar penalties for new city, conquering a city, growing your cities, any action that's supposed to grow your empire, however the penalty is more local due to different balance and numbers.
You may believe it or not, but Unhappiness penalties were increased in BNW (last expansion of civ5) for two reasons: wide approach was casually better in GK (previous expansion) and BNW was supposed to make late game more interesting (which required balancing out early conquest).
Though, this is again balance stuff I don't care about and history, therefore I am gonna redirect you to T-hawk's blog if you are interested in those aspects.
At this point I have the assumption that the fundamental flaw for an AAA title was this ANGRY RED unhappy face and COLOR_NEGATIVE_TEXT that was widely considered as a bad move and made a lot of people angry. Thankfully Firaxis grew up as a corporation to better manage players' moods with much more toned colours and tooltips.
And as for the snowballing, I consider that a rather fundamental mechanic of 4X games. Sure, there should be bounds, but the entire deal with expanding and exploiting is that you use your resources to get more resources, which allow you to get more resources, and so on.
The fundamental issue of 4X. They tried to keep a challenge in further stages of game which is important.
Imagine the alternative where you early conquer a neighbour: you are ahead in territory, production (not only per turn), population, culture, science. Technically it would mean that to keep a challenge other players would also have to conquer their surroundings.
What if other players would not conquer anyone though (not so uncommon for an AI)? We would probably end up in combat penalty vs AI on higher difficulties to keep at least a bit of challenge.
There's no reason to build a road other than to connect cities. So why not simply make roads free and do not add a bonus for connecting cities, just like it was in previous games? The fact that connecting cities gets you money implies you're supposed to make money from connecting cities.
Making roads free would end up the same as every previous iteration: road on every tile.
They introduced something new which eventually evolved into current traders.
The city connections were very complicated (/costly) to calculate and I like to believe that we have traders, because they decided to get rid off that. Even though adapting to builders seem a more likely cause.
Not sure if I'm a fan of it, faith tapers off at some point by it's nature, while you'd expect a tree to go to the end. Best to keep religious stuff in the civic tree imo.
Futuristic era could actually be easy.
Ideologies, secularism, cult of personality, there is some stuff to fill industrial/modern. It would probably overlap with civics, but currently civics also feel a bit overlapping with technologies.
Just a possibility I liked to think about.
Envoys are their own resource, meaning they don't compete with other things you can spend on, nor do you have the opportunity to gear your entire empire towards city states in order to absolutely dominate that aspect of the game. Which, in fact, also allowed you to win a ""diplomatic"" (economic) victory.
Decay of influence is just the city state begging for more money, I don't see what it adds to the game.
Both cases are about an extra layer of decision making.
Imagine how envoy dropping by 1 (each game era) or (after X turns since the last envoy was added) could affect the possibilites.
Though I am not saying any of those systems are better, they are just different.