Ive categorized all of the improvements into the following groups, and will then look over the cumulative base and tech bonus yields to see if changes are needed.
Marine Feature Group:
Beacon (on reefs/coral only)
A Lighthouse (upgrade from beacon)
Marine Preserve. This third one is not part of the upgrade chain, and is part of the natural wonders related additions. However it too can be built on normal reefs and Coral features, as well as the specialized natural wonder features. While still allowing it to be built on its own, my suggestion is to allow this to be the 3rd upgrade in the Marine Feature chain. I realize there is specialized python coding related to these which needs to be considered, especially in regard to combing them into a chain and incorporating the affected natural wonders.
Wildlife Chain:
Scavenging Camp (Wild): Feature only valid on specific fauna bonuses
Camp: This is the vanilla BtS camp. It is only valid on specific fauna bonuses. I suggest renaming this to "Hunting Camp"
Safari: I suggest renaming this to "Wildlife Sanctuary" and expanding to be valid on all Hunting Camp fauna instead of just elephants, and making this a the 3rd rung in the the upgrade chain. By the time this is available, those wild fauna bonuses should have much reduced importance.
Town Chain:
Cottage
Hamlet
Village
Town
I realize that the food bonus was added to encourage the AI to build them, but I never like that part of it. I'd rather give them an extra commerce. The other thing I think these should do is culture spread in staged amount up the chain. I would also like to use these in conjunction with buildings to generate free specialists which represent the bonus population obtain from these towns. Note that this may mean reducing free specialists from other building sources. Another idea is to add a 5th upgrade level "Suburbs" at the Consumerism tech. There might be a need for AI tweaking to give them other reasons for building these so that they are not ignored.
Feature Camps:
Jungle Camp
Desert Camp
Wetlands Camp
While some may have other opinions, I view these as a way to make use of marginal terrain. I think all need some more tweaking, Desert and Wetlands surely to be fleshed out more. I don't think Jungle Camp should have +3 production bonus from high level routes especially when they are the only improvements that does that. Other change ideas can be discussed later.
Wind Power Chain:
Windmill
Desert Windmill: (Change the name of this to 'Modern Windmill' or 'Wind Turbine', expand tits legal placement locations, make the windmill upgrade into it)
Seamill: Identical to Modern Windmill (maybe change its name to the same)
Windtrap: upgrade from Modern Windmill. Actually apart from making use of the graphic, I don't see much point in this upgrade. First if it upgrade the Land version of the modern windmill it should upgrade the sea version too. Second, the yield gains are small and can probably just be replicated with a tech yield bonus. I disagree with its ability to provide a food bonus.
The main purpose of the Windmill should be to enable the Windmill grain building via CV dependance in the city. Latter electrical power windmill should enable the wind farm power plants, possible giving power output based on the number of windmills in the CV, once the electricity property is worked out.
Factory Chain:
Workshop
Factory
Industrial Complex
I never build these, unless I have a city with no hills and no forests. Mines and Lumbermill generally out perform them, if perhaps they gave a free engineer like towns via a building dependency that might make them more attractive.
Fishing Vessels:
Fishing Boats
Harvest Kelp: Maybe rename to 'Kelp Harvesting Boats/Ships'
Whaling Boats
Whaling Ships
Not much to change here. We could reuse the Whaling Ships graphic and make a Fishing Ships upgrade for Fishing Boats.
Nature Preserves Chain
Forest Preserve (upgrades into Treefarm)
Treefarm (upgrades into Hybrid forest
Hybrid Forest (end of upgrade chain)
Nature Preserve: not art of the above chain but I group it with them because of similar function.
First of all I don't know what to do with Treefarm and Hybrid forest. They very late game improvements, but Treefarm seems redundant given that we can now do the whole Tree Nursery / Young forest thing. Does Hybrid forest even make sense anymore, that could not be accomplished with tech yield bonuses instead? Id it still desire? IMO Id drop Treefarm and Hybrid Forest. If you want to grow trees use the Tree nursery stuff. One thing that those 3 improvements did was have a positive iFeatureGrowth. Building them caused forests/jungles to spread. Nature Preserve doesn't, which is reasonable because it valid only on rock forms and other non vegetative features. I'd like to merge them into a a single "Nature Preserve" Improvement. It's main function would be commerce/ecology/Tourism and the like. Also no reason they could not help spread cactus and such. However to not end up with 2 improvements (Forest vs Nature) I would need to know how the code handles the iFeatureGrowth value. Will it still make a Rock form "grown" even though the Rock Form featureInfo definition does not have value for growth? I know I can test in a game, but I'd rather just look directly, but I am not sure what handles that yet. If it doesn't do it, I'd like to have it modify to only enhance the growth of a feature that has a positive growth value to begin with defined in its featureInfo.
The National Park Natural Wonder still shows to give +1 Free Specialist per Forest Preserve (which will upgrade out of existence) This could be modified into a general building to make these Nature Preserves more useful and desirable to build. (also an improvement CV dependency building.
Hydro Power Chain
Watermill
Hydro Dam
Hydro Dam obviously need more support and this basically follows the same ideas as the Wind Power chain.
Fruit/Plant/Grape Chain
Fruit Gathering Camp, Fruit Picking Camp, Orchards (should not remove features)
Plant Gathering Camp, Plant Picking Camp, Plantation (should remove features)
Grape Gathering Camp, Grape Picking Camp, Winery (should remove features)
First all of the above improvements require a bonus to be valid. Not all however show a bonus yield. IMO the base yield bonus should be dropped and changed to bonus yields. (This allows for finer, more varied control of what bonus gets what yield bonuses.) They can still have tech yield bonuses which apply universally. IMO if an improvement is ONLY valid on a bonus, then it should not have any base yields, and only rely on bonus yields and tech yields for its effects. This is the standard pattern, I'm not sure why some improvements departed form this.
Mining Chain:
Stone Tools Workshop
Mine
Shaft Mine
Modern Mine
Core Mine (Modern Mine current does not Upgrade to Core Mine, it should.)
Yield gain consistency up this chain needs to be tweaked some, as well as moving some base yields to bonus yields at the stone tools workshop level.
Quarry Chain:
Special Stone Workshop
Quarry
Same comments as mining chain. Possible add a Modern Quarry as an additional upgrade step.
Habitat Chain:
Small, Medium, Large, Huge
They are their own category, basically the same in function as the Town Chain, but I'm not going to address these now.
Pasture Chain
Scavenging Camp (Domesticates)
Nomadic Herd
Pasture
Nothing much to add here, they are all valid only on bonuses
Plant Forest Chain:
Tree Nursery
Young Forest
"Plant Forest" (fake improvement actually)
Why are Tree Nursery and Young Forest giving +2
To make the AI want to build them? They will take a step down when it turns into a forest which only gives +1
Farm Chain:
Seed Camp
Farm
Farmscraper: +30
, +5
? I know its late game but that seems excessive.
Farm doesn't upgrade into Farmscraper, should it?
Lumber Chain:
Wood Gatherer
Lumberjack ('Lumber/Logging Camp')
Lumbermill
Probably not many changes here, though I question the high yields for various routes. They are not as bad a jungle. I think Route yield gains should be mostly small and consistent across all improvements.
All other improvements are their own category. I am only mentioning ones I have comments on.
Moai statues (convert to a feature along with ancient temple and other similar structures. Make this a "Sacred Site" type of improvement instead, which later can be upgraded into the Archeology Site improvement.
Peat Cutter: production yield seems high at +3.
Tidal Harness: With above proposed changes it seems a weak version of Seamill I doubt it would even be built, unless it too allows a CV dependency building.
Solar Panels: Should also allow a CV dependent building.
Submerged Town: I think these should be named "Artificial Island". Function can be similar to the Town Chain.
Extraction Facility: Can be built on any coast or ocean terrain. Spawning Geothermal vents seem odd though, I imagine them as being more of a fixed thing. They can spawn methane ice also. I'm not sure if they should be available on all water, maybe just those bonuses. Should the 'Artificial Island' type improvement be more for all water tiles?