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If you're going with a 2D isometric view, then if the ground doesn't go all the way to the top of the screen, it's probably going to look all wrong regardless of what does show at the top. The ground only going so far up the screen before you get to the sky really only makes sense with a 3D perspective world. Or perhaps if you reach the end of the world and it looks like you could fall off.
If you only want to have so much of the game world on the screen at once, why not figure out how much that is and scale it up to cover the whole window? Or do you already have a bunch of artwork done that you can't scale?