A Huss jump 2 has been suggested by loads of people (not least myself) as perfect for DF, if themed to a giant tree. However, there have been plenty of reasons given by those more in the know than me as to why it will never happen. Including, but not limited to: it's too tall, and apparently the main section of the center support is too big to be driven round the roads in the area, and would have to be airlifted in!
As for flat rides not being marketable, when Chessington first got Rameses' Revenge, it was THE headline ride in the whole park, even more so than Vampire. I can remember the mad dash towards it at park opening. Within minutes, the whole queue would be full, and the temporary extention often stretched all the way past Terror Tomb, along the side of Dragon Falls, and right up to Magic Carpet (where Peking Heights is now).
So a good flat, with the right marketing, will be popular!