I haven't seen this idea but I think it would be an amazing feature. In RCT3 when you make custom terrain near the boundaries set for your park, only the land inside the boundaries changes. You can see why that would be kinda awkward; giant plateau that drops to a sheer cliff at the edge of the map. If it is possible, maybe we would be allowed to edit the terrain outside of park boundaries but not be allowed to build there. OR the terrain at the edge of the park would dynamically update to smooth the transition.