Idea is good but the thing about that magic and archer things are enhanced is more problematic since in RTS games mostly it's the terrain everywhere that effects your units. Such as a forest would make your units move slower and give them more hiding places wich could lead to a good ambush spot for caravans. But the thing you said about gaining more trade and so on in Harbor Cities that is quite more as you were thinking and is more what you were meaning.
I will not say anything with this signature... Except what I just said. And I will say the same thing when you read this over and over again.