I wouldn't mind the the option to unhide things (remove HD and FL), as this would be useful when replaying my own plays in order to better see how I missed things (as well as just finding out how on center I am (or other people are) with hidden circles). So I can certainly support it that far. I can't really see this as destroying the purpose of replays... the replay is still there in it's original form by default, this wouldn't change any of that. You'd just be able to watch someone's FL play, and if you wanted to, expose the hidden circles so you can see what they were hunting for before they got there. It's not like you're suddenly going to forget that the original player couldn't see the target of a cross screen jump, or that you couldn't just turn FL back on so you could see the original player's experience again. If you'd rather always watch replays with the mods, then just never hit the button.
But I can't see a point to turning off HR... it's too different from the actual play to really be useful or that interesting. DT/HT, I would see more along the lines of adding the fast forward button from auto plays to replays (along with a slow motion level)... useful, if you want to fast forward to a part later in a song, or look more closely at a certain part.