You may have already read it, but since I just did, I'll suggest Mousou Elektel or, more specifically, the accompanying DJ.
However, Spoiler (mouse over to view) with the way the main series seems to be progressing, all instances of fantasizing may fit your description!
Also, these might be a little bit too vague, as the given characters never specify that they were actually fantasizing, but if a statement like "I've always wanted to do this!" fits into what you're seeking, then there's the first story in Seinen wa Ai o Kou, the second story in Ijimetaihodo Aishiteru, and probably a bunch more!
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