My children, I come down from the Mountain with a tablet in each hand. Within each is inscribed a new rule that shall hopefully lay to rest a balancing issue that will keep making itself known otherwise.
As it stands, investments in trade are twice as profitable as Industry.
As such, I could either nerf trade value as I did before, OR, make it so Industry has extra perks. Industry already has the value of increasing 10% each turn, so every 10 gold is well worth it. Now, I am introducing more perks.
Rule Number One: Falling below the first world HDI average will incur revolt risk. Your HDI is your Industry, which represents GDP per capita at the end of the day. Yes this is a butchering of stats, but foxy doesn't care.

Your people are pampered and expect that a first world country will be able to give them a good quality of life. Provided you have low instability, the higher RR will just mean constant tax lowering, but as the instability grows...
Abridged: What this means is someone in the lead like London now has -8% RR from being so prosperous; their people are content and happy with the government's economic management. Victoria's people, by contrast, have +7% RR now, they are angry at the government's management.
Ruler Number Two: Your Industry value caps your military/espionage value. It's easy to follow: 0.0300 Industry means you can have 30 in any field, 0.0200 means you can have 20, etc. This only applies to spending; points can still be gained through experience. Logic: A third world nation could not hope to field an army as well-trained or well-equipped as the US, German, UK, etc. armies.
Rule Number Three: A state's industry can at most double each turn. This is to prevent the exchange rates system from resulting in hilarious stupidity. This is for humans' benefit as well - it keeps minor nations from becoming economic giants.