Fungles would definitely be a much better place for a dark ride as it's more concealed, could blend in better plus no one would see the back of it (the shed) unless you use monks walk and the Blair witch is there.
Using the arena for a dark ride wouldn't be as good as Fungles site because 1/ the park has lost potential space to hold yearly events (e.g Sun scream & Easter) 2/ they will have to relocate one of their scare mazes elsewhere (which for somewhere like Thorpe won't be easiest) 3/ a big metal building in the middle of the park will be a bit of an eyesore, obstruct park views and in general not be great for the layout.
Despite being 10 years (almost) since the last flat installment, Thorpe doesn't need another for now but is in much need of some dark rides/immersive experiences. Towers has Hex, Duel, Sub_terra (among others) and Chessington has Tomb, Bubbleworks, Hocus and Zufari. Thorpe's only immersive experiences so far are tidal wave, Swarm and Angrybirds 4D and even that is really pushing the boat out.