I don't think I will have a theme at first so I can just enjoy playing the game opposed to always searching for certain things!
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I doubt that they will allow for you to customize your terrain (except for the usual such as planting trees, flowers and bushes manually) as that is part of the feel in tradition animal crossing games of having a set town landscape and working with it (such as new leaf where some towns have a southern, eastern or western facing waterfall, or a mini island next to the waterfall). As for sky customization, could you tell mw what that is since I am not familiar with it? If you mean like changing the sky color or weather, it will most likely not happen as animal crossing weather is supposed to be random like in real life, and sky color is supposed to be realistic. Other than those, though, New Horizons will have plenty of customization which will allow players to create their dream island!