If you are using that method:
Is Hogwarts Express going to better or as good as Forbidden Journey or Gringotts? Probably not. So no E-ticket there.
Is Hogwarts Express going to be better than Hippogriff? Yes, so not a C-ticket.
So IMO it is a D-ticket. Sure it will be a cool and awesome experience, but not anything thrilling or setting the bar for themed attractions. It will be like a calmer version of Spiderman or Transformers. Even if the developers are calling it an E-ticket, that doesn't really mean anything. It just means they are trying to build hype for the attractions like they are supposed to.
Same thing with Seven Dwarfs. It is a D-ticket, and NF won't be as good as the Harry Potter expansion because it has no E-ticket. HP will have two rides better or just as good as Seven Dwarfs, so the area will be better.
I'm not trying to diss HP or anything, I just can't see Hogwarts Express being an E-ticket. That doesn't mean it won't be a great attraction though.