There is some balancing to be done. I think they should increase the benefits more than reduce the build cost. Its important that you have to prioritize and that you cannot build everything.

In a game of balancing priorities, I find the concept of +something when idle is rather hopeless. If a city is being idle and run out of stuff to do, I think that the game dont have enough buildings, resources, units or whatever to produce. I dont want to calculate in my head how much research I miss the next 5 turns(+25% research when idle), cause I want to make a pioneer. Its quite hard to track these effects too, without going into the city and look at the buildings. 

I just find it very annoying and will not produce such buildings or chose them at level up of cities. EVER. 

When I get the choice between a building giving +25% research bonus when idle or a +1 research pr turn building, its a pretty difficult calculation. I just dont like the concept. I think we have to agree to disagree.

Not building something= boring.

Running out of buildings to build= boring.

I don't mind buildings having marginal effects- not every building has to be equal.

Maybe if your entire empire consisted of one city and you never built outposts or did anything else besides sit on your little city... Run out of buildings? Get a new city. Adding buildings just to click on is tedious and doesn't contribute anything. Upgrading outposts/resources is more interesting than going through an endless cycle of buildings with slightly better effects since you have to first claim/clear them and then develop their protection.

Really? Those are my favourite improvements. Perfect for starting a city with high production, or catching up later in the game. Plus it is rarely just that production you gain, since improvements all over the place reduce building time and training time. (increasing the value of production).

Also, the fact that they scale with materials really highlights how important materials is in the game. I never settle in a spot with less than 3 materials anymore, and preferably there should be nearby woods a swell.

This is a good point.  Personally, I think a choice between doing something and doing nothing is generally not a fun choice.  Fun choices are usually picking between doing thing A or thing B (or thing C, etc.), allocating scare resources, building up a powerful champion, etc. 

With the right structure around it, I can see how it could work.

But IMHO with the structure of the current game as it is, idling my cities is not something I'd even consider a choice.  It's something that happens by accident, or because I have run out of things to build in that city.  Really, if I am in a position to idle one of my a major cities it means that I am so dominant that maximizing production no longer matters; and what is the benefit of the idle bonus at that point?

Let's do some math!

On a good day something like Timbermill gives you +3 Production per turn. It costs 387 Production to build. That means it will take ~129 turns to compensate for the opportunity cost. If most games last 500 turns by the time the game releases (currently more like 300), you will only see 270 turns of gain, which is only 810 extra production overall, if you get the recquisite techs by turn 100. Considering that earlier turns are more significant in terms of opportunity cost, this is a bad decision, only offset by the AI's tendency to not do the math as much as the player.

The best solution would be to make production increasing buildings cost much less and do much more. Reduce the standard Material yields to offset the increase. Mason and Mill are actually worse because they have higher costs and only benefit construction.

I think the game should be shorter. High production costs are delaying the game without reason. Better to play short shining game, than overextended game. Else it leads to many unfinished games. 100-200 turns are enough for this game. It is more suitable for multiplayer. Production and research costs can be decreased to achieve this.

Consulates are WAY OP.

Considering that you can build outposts practically anywhere, the cost of a pioneer + consulate <<<<<< +1 growth.

0.5 would still be OP. 

0.25 ... maybe...

It should be zero, it's a silly thing.

Okay, let's do some math. A Lumber Yard adds 3 per material (a Timber Mill adds 4 production per material btw).

Let's say a city has 3 material, that directly translates to 9 production per turn. The Lumber Yard takes 176 production, which means without any other modifiers you'll need ~20 turns to complete it using only its own production (ze opportunity cost).

If your game lasts 500 turns, you'll earn 480*9 production throughout the game. Obviously I'm not including secondary benefits such as scaling with Unit Training Discount and Improvement Construction Cost Discounts.

Okay, let's do some math. A Logging Camp adds 2 per material (a Timber Mill adds 4 production per material btw).

Let's say a city has 3 material, that directly translates to 6 production per turn. The Lumber Yard takes 80production, which means without any other modifiers you'll need ~13.333 turns to complete it using only its own production (ze opportunity cost).

If your game lasts 500 turns, you'll earn 486*6 production throughout the game. Obviously I'm not including secondary benefits such as scaling with Unit Training Discount and Improvement Construction Cost Discounts.

Let's say you do the upgrade to Lumber Yard at turn 100 in the game. Lumber Yard adds 1 more PPM and takes 176 production to build.

At 3 materials, the paid opportunity cost is ~59 turns. That leaves you with (500(-100-59)) * 3 production net benefit at the end of the game (comes out to just over 1k).

Production boosters are brutally strong at the moment.

1) I only used a 3 material city. I frequently get 4 material cities and even 5 with a clay mine.

2) Set in stone multiplies any bonus, and scales perfectly with

3) Buildings that enhance the production value further, such as Barracks or Mill

My advice - stop treating production like something you get for free. There are huge bonuses to be picked up, and they'll benefit you more the earlier you build them.