Nothing more intricate or "deep" is needed - BNW is leaps and bounds ahead of past Civ games in actually having economic management rather than just having commerce as a resource that magically manifests in the landscape and, for some reason, needs citizens to actively work the roads (what, most money's coming from toll booths?) - you actually have to build up an economy differently from managing every other resource. You even have actual expenditures to worry about and that you can actively manage by choosing where and what to build rather than artificial losses of income due to arbitrary maintenance or corruption calculations. Specialising a city around economic activity, and as a trade hub, adds a new 'type' to the typical two or three archetypes of Civ city specialisation (which previously amounted to food-specialising 'Great People Factories', culture cities, and production hubs - only two of which have any real-world precedent).
It also has, in principle, strategic resource limitations, but in practice a combination of overabundance of these resources and too few units and buildings that make use of them (and the lack of Civ IV-style units that require multiple resources) works against this as a practical strategic consideration.
But what it could do with is more detail to better-exploit this improved economic model, as well as incorporate missing features like tax rates, present in nearly all games of this type (and usually trading economy against public order in some form rather than prior Civ game's conceptually bizarre money vs. science trade-off).
- Reintroduce Civilization Health.
Health would function as a basic level of your populations efficiency, if you're people are sick, have low life expectancy, or are starving from military sieges than the effectiveness of each unit of population is going to suffer. Farmers, miners, merchants, soldiers etc are all going to do their job less effectively. Health can be promoted by buildings such as aqueducts, supplying fresh water and in the later part of the game building hospitals & funding public health programs.
I've seen a Civ V health mod that does a creditable job; the downside is that it focuses on simply translating Civ IV health to a Civ V concept and, while it solves some of the problems, Civ IV did not have a terribly good health system - arbitrary population limits that made it a management chore instead of rewards/penalties that scaled with your ability to manage it (i.e. greater penalties the lower health is, rather than a flat rate penalty that's the same at -1 health as at -10, and bonuses that encourage you to maximise health rather than just stay 1 point ahead of unhealthiness), and ways to generate health that mostly rewarded you for what you were doing anyway (removing jungles, settling forests, connecting resources, building granaries and grocers). Plus, as you say, unhealthiness tends to affect production, not growth rates, so a food penalty (or indeed bonus) from health effects is not satisfactory.
The mod is also hampered by being twinned with a disease system that doesn't work very well.
Moral of the story: health can certainly add to the Civ V experience, as it did in the mod, and work within a Civ V framework, but it needs to be something other than a translated Civ IV system.
As your civ progresses into the industrialised/information ages health becomes increasingly important as it is required to maintain a population of professional workers, poor Civ health will seriously reduce your scientific development & reduce happiness.
All of this is very nice as a descriptive, but it's not going to be informative if everyone agrees in principle (which is likely) but there's no clear way of translating this to mechanics. A science and production penalty would definitely be a good way to reflect health, but how to determine when a city is healthy or unhealthy within the framework of what is, fundamentally, a mechanically very simple series of games? A 'health resource', as attempted by Civ IV and the Civ V mod, is a pretty shoddy approach and won't be able to represent most of what you're asking for, but it does have the advantage of fitting into a Civ game framework (which has been described as 'filling buckets' of resources - by a reviewer intending that as a criticism of Civ V, but it's fundamentally the way Civ games work).
- Make Happiness more realistic & dynamic
Currently happiness reduces & thus stagnates population growth, decreases the effectiveness of soldiers & eventually causes barb rebellions to appear - but that's it.
That is, however, a pretty big "it", particularly when you throw in later-game impacts on science (science bonus from being happy in Rationalism). One thing I'd like to see is the happiness-culture link from certain social policies made part of the basic happiness system (i.e. excess happiness contributes to culture). The Golden Age system is intended as a reward for maximising happiness, but is not generally considered worth building strategies to maximise happiness around - most players will still play at parity or close to it for most of the game.
It can be crippling but its not all that interesting.
I don't see why happiness should reduce growth (For instance - the most unhappy countries in the world in Africa and the middle-east often have very high birth rates)
Because happiness in Civ V is a conflation of the happiness and health systems from past games, and Civ IV health was the mechanic that stalled growth. That, and population is the most important resource in the game, so in a game context it's the biggest penalty mismanaging happiness can provide. Happiness is meant to be the game's main management resource, so you want the effect of mismanagement to be serious - having it cost gold, say, when you're already swimming in gold would make it of trivial relevance to gameplay.
It's the same reason for the science vs. gold slider of past games - as I mentioned, this makes no actual sense in real-world terms, however science was the most important resource that could be traded off against covering maintenance costs and so forcing the player to use the slider as a management tool.
I would make unhappiness have an exponential effect in actually increasing population growth (which requires more food) but also reducing the food and hammer yield of tiles.
Sorry, but this is a terrible idea. You don't seem to be looking at the game in the context that it is, indeed, a game. If you were to implement something like that, the optimal strategy would be to maintain an unhappy empire because population is everything and more population lets you work more tiles, more than offsetting any penalty you can realistically impose on tile output.
In this, the past Civ games had a better model: unhappy citizens simply won't work tiles at all. Of course, this is only easy to work if happiness is calculated at a local rather than a global scale, so that the system can decide (and the player can quickly see, and most importantly control) which cities will suffer the unhappiness burden.
An obvious solution then is simply to revert to the Civ I-IV happiness model and ditch global happiness - with a few tweaks to the way the economic system works the economy alone is enough to constrain expansion.
So ultimately if unhappiness is not addressed you're people largely stop working efficiently, population grows out of control and civilians end up starving to death (which will have a further effect in reducing happiness).
Populations both growing and starving to death will however cancel one another out, so you're back to the penalty that people stop working. With the above solution, since this is exactly what the previous games did.
-Reform the existing trade-luxury options.
Right now it gets really clunky having to initiate trade deals with every Civ separately. There should be a 'Global Market' UI feature that lets you put surplus luxuries etc on a world market for Civs to buy. You would be able to get a see information on your relations with each Civ so you can refuse to trade with a Civ you don't like. You can also set a price that you are willing to sell a luxury for. An advisor would tell you the proper price for the luxury so you know what to sell it for. There would also be an option to sell at "market value".
In other words it makes it easier to buy & sell luxuries (all done on 1 UI screen, rather than having to cycle through multiple Civilizations which gets very tedious)
I think lux trading is one of those holdover systems that isn't really needed with the BNW mechanics (just like global happiness). Just change the way luxuries from trade add to trade income, and remove lux trading altogether. Aside from anything else, the lux-for-gold trading system has consistently proved to be exploitable, such that the designers have restricted it since release (the lump sum gold only with DoF restriction); removing it altogether means there's no need for this kind of patching (not that I mind lump sum gold from DoFs only, which is a good incentive to form DoFs in other contexts than lux trading).
For instance, the system could work this way:
- +1 gold for every lux the other city doesn't have if you're the exporter.
- x2 gold output from trading this lux if the lux is connected to your city by a road.
- 'Monopoly' modifier (perhaps another x2?) if your trade route is that city's only source of that luxury (i.e. if it does not have a city connection or a trade route to another city with that lux; luxes will only be counted for city connections if they are connected to the city by road).
- +1 local happiness in a city for every lux it obtains via a trade route (replaces the happiness bonus from getting luxes in trades) from an importing city.
- Each copy of a lux the city has can only be traded to one city at a time (on a first come, first serve basis)
Strategic resources could of course be revised on similar grounds, with access to the resources provided by trade routes only rather than by Askia turning up at Darius' court pulling a load of wagons filled with iron for trade. Instead, in diplomacy you could exchange or bargain for trade rights, which would allow you to set up routes with that civ's cities (and could be revoked/not renewed, so that a civ doesn't have to go to war or pass an embargo in the World Congress to stop you trading).
-More resources (both strategic & luxuries)
We need tea, coffee, cocoa, tobacco & opium.
Move gold, silver & copper to bonus resources
Why bonus resources? Aside from anything else, as the game stands bonus resources don't add value to trade routes, and it would be especially unrealistic for these resources to have no value as trade goods.
More resources of all types should have effects similar to marble's. Iron, for instance, could improve production speed for melee units; copper could boost research into Electronics; gold and silver might be worth more when traded (x2 resource diversity bonus for that resource when part of a trade route). More resources aren't needed because that adds nothing strategically - what is needed is resources that have different effects, and at different game stages, that are likely both to make different resources more or less valuable to settle depending on context (for example, with the above examples gold would only be of particular value in a trade city), and to prompt conflicts over obtaining them. It's a big flaw with Civ games, probably born out of the fact that they're still based on a game design that didn't have resources (those were only introduced in Civ III), that resources are very rarely valuable enough to be worth fighting over (except some strategic resources, and in those cases it's the civ that has them which tends to have the military advantage precisely because it has them), and yet this is one of the major sources of real-world conflict.
-Some resources play a more intricate part of your functioning economy
For example once you become an industrialised nation the supply of coal becomes very important & you need it to effectively run factories
All that's needed here is a tweak to an oversight in the existing mechanics, whereby losing access to a resource doesn't have any effect on buildings that have already been constructed. Just remove those buildings' bonuses until the supply is re-established (which I think is how Civ IV handled it). More buildings relying on specific resources are also needed, because buildings are the only things that are relevant throughout the game; as it is iron etc. have short useful lifetimes because their units obsolete fairly quickly.
-Introduce a Civilization wide taxation policy
This affects the amount of income you can tax from each citizen. More tax can ultimately stifle research & and reduce happiness but if you are smart in reinvesting it into your economy it can work - i.e. Scandinavian economies
A happiness/tax trade-off is needed; stifling research is nonsense since in most of the world research is state-funded, and increased taxation means more state income that can be devoted to science.
- where one Once city can be the "Bread Basket" were you attract farmers to it. These would be small population cities with extensive farmers fields. Of course trade routs need to be able to transport food from one city to another.
Another becomes a Mining town with extensive mines in surrounding lands. These would large high population cities. again hammers should be able to be traded to other cities, perhaps in exchange of food or gold.
Another city can be a center of commerce and trade. acting as channels of trade, generating gold and culture.
Another city can be can be a city of industry, creating more advance materials from simpler ones, and a center of trade.
The one feature I've seen in a mod that I really want to see implemented in Civ is population migration. Otherwise the above is mostly how Civ already works on higher difficulties - you do generally need to specialise, and trade routes have facilitated this by allowing you to focus on high production, say, and still supply food from elsewhere.
EDIT: Having said all of the above, as I notice an early poster on the first page pointed out, Civ V now has a pretty good basic economic model since it was a focus of BNW, and the economy is therefore not the major game system in need of change.