- adding a 'local luxury' bonus for each city, +1 for each unique improved luxury around a city (doesn't care about luxuries owned by other cities)
Just confirming, as I believe this is how it works.
1) If I have gold on City 1, City 1 = +1 happy. If City 1 has two gold: City 1 = +1 happy (aka no additional bonus)
2) If City 2 has a gold, also +1 bonus (City 1 and 2 both get a happy for each having a copy of gold).
3) Luxuries acquired through trade only affect the luxury scaler. Which from my experience so far means acquiring new luxuries provides almost 0 benefit.
What if for the Global Number, each luxury added to the denominator with a scaler.
Approach 1: Global Happiness = 1 - (Unhappy / [Total Pop + Era Scaler * Unique Luxuries]).
So luxuries would help keep the Global Happiness up, and could scale with Era to keep them relevant as your population increased. By affecting the denominator instead of the numerator, it scales a little more elegantly (affecting the numerator would probably shift the numbers too strongly).
If we went this approach we would likely need to adjust the Global Happiness ratios (probably should do that anyway they are very forgiving right now except in the very earliest part of the game). But it would help to make luxuries relevant for Trade again.
Approach 2: Another option is you modify the population by a percentage based on the number of luxuries.
Example (luxury adds 10% to Global Population): Global Happiness = 1 - (Unhappy / [Total Pop * Unique Luxuries * 1.10])
So this adds an automatic scaler that might get stronger in the late game but would be softer in the early game compared to approach 1.