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Well, the very idea of tying the city screen to the terrain is actually quite good, especially since it was already being considered. In the fifth installment, there's also a departure from the norm when you enter the city screen of a human castle in an underground location, or the undead on a green lawn. And I appreciate that they tried to do this in the fourth game. But because of this, the aesthetics and atmosphere suffered greatly. Another question is whether it will be possible to maintain the atmosphere if the city screen changes depending on the terrain, and whether it's even worth it. I'm not particularly bothered by the mismatch between the terrain outside and inside the city; I get immense aesthetic pleasure from the city screens in parts 2, 3, and 5, and I don't really think about how inappropriate such landscapes are in the city when compared to the adventure map.
In Disciples, by the way, this problem was solved radically; they don't paint the capital to match the terrain type, but rather the terrain to match the capital type, so demons will always be in lava, dwarves in snow, elves in an autumn forest, and so on. It's even more amusing when you manage to override the enemy's terrain color and fill everything with your own terrain type; capitals on foreign soil look very alien and out of place. But in Disciples, the terrain color is an important element of the gameplay, while in Heroes, it's more of a cosmetic feature. There are concepts of movement penalties and native terrain, but there's no such fundamental and key influence on the game itself.