Does it HAVE to be something offered by the industry in the public domain now to be a suitable addition?
For all we know, Merlin could be doing deals with manufacturers like Tussauds had with B&M and Towers all those years ago which gave us Oblivion and Air, when prior there was nothing like them before (ok, Vekoma had the Flying Dutchman, but Air's technology exceeded that version greatly)... Same thing for Thirteen really...
Nothing stopping the park being imaginative and getting something 'under the table' as it were...