Love the mockup!
And I agree that this would be good (at least most of the times that I think about it ).
From what we’ve seen so far the only way for the little dudes to get to another level of the landscape has been by using stairs, and that would just be weird if your out exploring the wilderness. It could be done using smaller cubes that make natural stairs, but I don’t thing you want too much of these high resolution areas, so things like soft, rolling hills would either have too high steps to look soft or be to high res so that they just look messy.
If you define the art style as blocky then yes, slopes could mess that up. However, if you define the art style as retro then it would be ok, since many old isometric 2d games use them. The landscape doesn’t have to follow the exact same rules as the rest of the models.
Three “slope tiles” would be enough, to keep the simple style. A straight slope (one side up), an inner corner slope (2 sides up) and an outer corner slope (one corner up).
Oh and I second the “almost guarantee” that the world generator has nothing to do with Qubicle.