As usual I'll take middle ground, I see perks both ways.
One of the things I liked before was the way it needed 2 huge buildings to build the monopoly. Although it was buggy and didn't actually work, it meant you had to plan carefully where your specialist centres were, as it was rare a city had 5 of a single resource. That said, it's also nice to have buildings that don't consume your resources (since they contribute to your happiness), and I think the reward of the monopoly is nicer with the bonus per tile rather than +800 production. I think a balance is needed. Also I think that having all the buildings available immediately was not a good plan. There is a lot that can be done with that
My suggestion:
Resource consumption:
Small and medium buildings should not consume: only require that there is an improved tile worked by the city. This means that you can always build these levels of the building. The Large and Huge buildings should require 1 of the resource. Since it's rare to have more than 1 in a single city this means you have to choose where to specialise for Huge buildings, or go for global larges. The monopoly should need 3 of the resource, plus 3 huge buildings (a total of 9 consumed including the larges)
Techs:
Smalls should be available early of course. Mediums and large I think should be stuck onto Trade fairs. Huge and Monopoly should be a while later, something renaissance/industrial? I don't remember that part of the tech tree by name so well, usually by then I just set a general target and let the tree work itself. This gives time to appreciate each level, and gives more freedom to balance the buildings.
Rewards:
I prefer the Food Gold Prod (FGP) per resource in a way over the flat bonus, but a combination of the two is possible. The balance of the different resources giving different ratios of FGP was quite good, but later buildings should add a touch of gold too to symbolise the increasing profitability. The huge building should reimburse 1 happiness too, but the large should stay as costing a happiness by consuming the resource. I think the following is roughly balanced with the existing tech requirement suggestions
Small : +2 FGP
Medium : +1 FGP, +1G +2 FGP per tile (ones which are split have to priorities one of them)
Large: +3 FGP, +1G +2 FGP per tile (split ones can use the other)
Huge: +3 FGP, +1 Happiness, +3 FGP +1G and +1 culture per tile
Monopoly: +3 FGP +2G per tile in all cities and a golden age
This gives the following totals:
Small : +2 FGP
Medium : +3 FGP, +1G +2 FGP per tile
Large : +6 FGP, +2G +4 FGP per tile, -1 happiness (-5 if it's the last of the resource)
Huge : +9 FGP, +3G +7 FGP per tile, +1 culture per tile, -1 happiness (-4 if it's the last of the resource)
Normally you can expect that only one of each tile is in each city:
Small : +2 FGP
Medium : +5 FGP, +1G
Large : +10 FGP, +2G, -1 happiness (-5 if it's the last of the resource)
Huge : +16 FGP, +3G, +1 culture, -1 happiness (-4 if it's the last of the resource)
For 4 buildings that isn't that unbalanced, but is quite substantial, especialy since most cities can get at least the first 2 for at least 1 resource. It's not much difference to the market/bank/stockexchange buildings. To turn this into an example for wheat:
Small = +2 food
Medium = +1 food, +1 gold and +2 food per wheat resource. Requires small building
Large = +3 Food, Each improved wheat resources worked by the city gives an extra +1 gold and + 2 food, requires 1 Wheat and medium building
Huge = +3 Food, +1 happiness, Each improved wheat resources worked by the city gives an extra +1 gold, +3 food and +1 Luxury, requires 1 Wheat and large building
Monopoly = Each improved wheat resources in every city gives an extra +2 gold, +3 food. Requires 3 Wheat and 3 Huge buildings.
Base wheat on farmland is 5F 2G 1P when farmed? when you finish and are getting 15F 7G 1P for that wheat tile you will really appreciate it!
Strategic resources:
It was nice having something on them, but they can't be the same balance. I'd say just 3 levels: small (consumes 1), large (consumes 2), monopoly (consumes 5). mostly they could be about the same levels otherwise
Trading:
It's not a mistake that over half the bonus i've suggested is per tile. It would be nice to have all the resources as luxuries and be tradable, so you can invest in Huges in more places by trading like mad, but you will not get such a big return unless you have the tiles yourself and choose where to put the facilities. I don't know if it's possible to keep track though of consumption: If you have 1 of a resource in two cities, build up to huge in one, trade 2 in and build up to huge in the other and the trade deal finishes you should have to trade in 5 to get the monopoly, not 3 (and that is not an easy trade to make, you'd have to give away half an empire!). I don't know how the engine works for that. Otherwise we should have everything back to non tradable. You have to get them on your own!
Naming:
Small, medium, large, huge: functional but not immersive. I'd suggest:
Small -> XXX Ranch / XXX Smelter / XXX Farm
Medium -> XXX Depot / XXX Market / XXX Trader
Large -> XXX Distribution Facility
Huge -> XXX Global Corporation
Monopoly -> XXX Monopoly
I think it better reflects the evolution from being a small local building, to being known as a local speciality, to being increasingly commercialised, building private company structures and finally having a capitalist monopoly over it worldwide.
Great Investments:
Something that we don't see elsewhere but could be very interesting to add. A side building on the chain, only requiring the small building but giving huge benefits. The idea is to have a building in the ancient era that has requirements comparable to modern era but also has the same sort of benefits. Ancient techniques that require huge effort rather than high tech. Carefully nurturing the soil with the right composts to make it a little more ideal for that crop, carefully breeding that animal and controlling the population, eliminating any natural predators, The sort of projects that represent a hundred years of careful work. that it would take a hundred turns for even a capital city of the era to build reflects that investment. The hard part is the balance, and it depends on if it is a national wonder or a regular but expensive building. Things I would consider as rewards would be >6 FPG if a building, 2 extra of that resource if as a wonder, making the resource give +2 culture/science (for all cities if as a wonder), etc. Granted a later civilisation could make this more easily, but having it available early but at the cost of any other production for 100 years is an interesting option for the player.
mischief
My suggestion is just ideas which I think are balanced and technically feasible, feel free to adapt them as you see fit.
