I think it's going to be a cross valley coaster and will come in 2016.
My glass is - currently - half full!
I do wonder when they'll replace the Flume though.
If it was coming in 2016 then plans would already have been submitted. I dare say we may also have seen signs of construction starting.
I'm expecting plans to come out later this year ready for opening in 2017.
Just to play Devils advocate, depending on its height, location etc we may not see plans until last minute as not all installations need a planning application. Although a cross valley would definitely be subject to an awful lot of planning restrictions and conditions.
Dan is right though, I think we'll see a planning application submitted at some point this year, and most likely some Trademark registrations mid 2016 which will give us an idea of its name and possibly theme. 2017 fits perfectly with the long term development plan and allows Merlin to maximise attendance at Thorpe next year for its huge development and then Alton Towers the following year. Expect to see them pushing Merlin Annual Pass sales too once SW8 is somewhat announced.