Island maps, instead of a landmass surrounding the park, it would be nice to have a mass of water surrounding the park.
There is a lot of potential for creating some awesome parks surrounded by water.
From what I remember about RCT3 though, it looked kind of bad when you did that.
Part of me was disappointed when the Tropical setting for sandbox wasn't like this, though. It's odd because I'd imagine having the surrounding scenery of an island-based park would be a lot easier to craft up than the elaborate hills we usually have.
I certainly would love to see such a setting included.