Improvements! (That's what she said!)
I lack the ability of trading maps, but also understand the problem that you can see all resources, forests and natural wonders.
So why not introduce a second layer of dim layer of fog, where you can only see if it's a hill, plain or mountain. And of course cityborders and/or cities of other players.
So we dont get these senarios:
Babylon: "Help me with the war on Ceasar"
Player: "Alright, where are they then?"
And if you ever introduce Vikings in the game, give them double speed when walking along river tiles. This would emulate the real vikings ability to rapidly move along rivers due to their longships having a low hull to enter shallow waters. It works well with your gamelogic considering the Iriqoi special trait. But since the iroqui also have trade routes through forests, giving the Vikings the additional bonus of being able to move directly after landing from water would be a nice compensation. This would PERFECTLY simulate real world Vikings.