"异想天开"，汉语成语，比喻想象着荒唐离奇的事，或提出暂时难以实现的想法，可以翻译为“imagine something impossible”。还可以用英文俚语“ask for the moon”表示，意思是“to make requests or demands that are extraordinary or unreasonable”。
He asked his dad to buy him a yacht as a birthday present. I think he was asking for the moon.
Don't ask for the moon. Be reasonable!