I agree that a prequel that doesn't overlap at all with the action of the original story doesn't really seem like a remix. And that a POV shift that doesn't reveal anything new, while qualifying as a remix, is a bit dull. Tone and setting shifts are usually fun. One thing I've wanted to do, but haven't yet, is try to rewrite a magical/supernatural trope story in a straight realism context. Maybe I'll get the chance sometime later!

I might add that I've never done the kind of remix challenge where you choose the story you're going to rewrite out of an author's body of work, only the kind where you get assigned a specific story. There's something about working within those constraints that I find to be a lot of fun.