I'm not sure there's enough space to put something interesting in the back area of the park. They could run an out-and-back hyper along where the Loggers drop is, but I'm not sure where it would end up after that with all the water, buildings and Inferno if they continue alongside the park boundary. Building out on the water would push the cost up and make planning harder with the flood assessments.
A GCI woodie is possible if Merlin bought on bulk again. But they may have thought it too big a risk to have two in the UK, especially if Wicker Man had disappointed. But if they did go for one, my guess is it would be a faster and rougher ride so it doesn't get labelled as a family coaster, as I don't see Thorpe putting a big investment into the family audience.
But we should be seeing planning permissions going up soon, as Wicker Man's went up in May 2016.
My money is on a B&M hyper. They need high capacity and high reliability. But saying that, Merlin will surely add an RMC in somewhere soon. I could imagine a 3 park deal of Thorpe, Heide and Garldaland being made to try prevent other European parks getting ahead on the RMC hype.