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のに is an adversative conjunction, and it expresses a wish which is impossible/hard to be realized when it's put at the end of a sentence. Thus, it's close to subjunctive past in English.
空を飛べたらいいのに(でも実際は飛べない)。
I wish I could fly (but actually I can't).
It's for just ~たらいい. のに shows it's impossible or hard to be realized, as I wrote.So this: 空を飛べたらいいのに
Is meant to be interpreted as:
空を飛べたら→if I fly in the sky, いいのに, it is good.