I really hope JK changes her "no Voldemort" policy. He is the "Darth Vader" of the HP franchise and it would be a shame if he didn't make an appearance even if it was just a cameo or something.
I honsestly feel like he should have been apart of the Forbidden Journey. It would've given the ride more of a story. Like he was so angry that muggles were allowed inside Hogwarts that he sent Dementors to suck our souls and then made an appearance at the end where Harry fights him. It would definitely clash with the stories and movies (simply because it doesn't happen) but it would've been awesome. Oh well, FJ is still my favorite ride.
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Also, I remember in the books it said that Dementors weren't visible to Muggles. I think it was in the fifth book where Dementors attack Harry and his cousin when he's at the Dursley's. But yet we are able to see them in the ride.