I don't remember this idea having been given before, so that may be a reason why it hasn't been implemented
The designers lurk here often, so I don't think you are spitting in the wind. But you may prefer to write a specific thread with an obvious thread-title in the Conquests forum (for them to have more chance to read), or to speak about it when a chat with Firaxians happen.
I think it is a great idea, and I think Firaxians will think of it as a great idea too. The problem with Civ's difficuty settings is that the end game is always easier than the early game if you managed to expand yourself.
In Conquests, the Firaxians have tried to adress it somehow with the Feudalism government (which boosts a Civ having small holdings), but your idea will go more drastically in that direction