Well, part of it anyway, I don't think it's technically owned by anyone but after years of having so many different attractions and very few staying permanently (bar really, the Aquarium and Namco long term), I wouldn't be surprised if Lambeth Council will be happy to take more of Merlin's money...
As for what it could be, the only brand that isn't there atm Midway wise is a Discovery Centre... Whether that's a viable concept due to being near Legoland (even though this will be sold towards day tourists in the centre, which is fair enough, the others are) is arguable, but unless Merlin are using their imaginations for a completely new brand then that's my bet...