Buying and selling units.

Buying and selling tech.

Visual representación of volume traded.

Chart showing everyones yields to compare global output. Perhaps discovered with a technology.

Excess science goes towards gold or production when no tech is available.

Great people (new slot for geniuses, produced when you have 10x or more of the same type at a progressive pace)

Workers have cultural traías. Eg. If I take a phoenician city, their marchants will give +2. Or if I'm harappans and steal greeks. Those come more science. Stability penalties too.

It would be perfect if they could bring the trading system of the Endless Legend to humankind

this looks great!