Not necessarily an economy bonus, but the strongest civs are the one with powerful units/bonus and a mean to reach them.
For example Mongol have good hunters to reach faster feudal age, bizzantines got cheaper trash and stronger buildings to reach imp and their powerful cataphracts and so on.
The point is that a civ can be more or less flexible but everyone need something consistent that help reach their potential.
I have an idea for a new powerful eco bonus.
Every time you research a TC technology you also train a villager for free.
Not as strong as the Viking eco bonus, but would put you ahead a few vills in a staggered way, for a max total of 5:
After Dark Age: 1 Vill (loom)
After Feudal Age: 2-3 Vills (wheelbarrow, town watch)
After Castle Age: 4-5 Vills (hand card, town patrol).
I would love to see this bonus in the game!