I'm thinking an upgrade for X84/X85 is most likely, thought I'm not sure they justify deckers - unless the vehicles are cascaded Mercs or other single decker.
I can't see the X7 getting newer vehicles - unless of course, it is been increased in frequency even just to Peterlee or Station Town, to try and rival the X21 even more? If it is for X7, it'll have to be Mercs.
Mind if it is Mercs, I wonder if they are been used as spares, rather than allocated to a new service? One could argue having 5337/5338 at Stanley has been successful on the diamond, and having some spare Mercs at Deptford/Riverside, may be a good idea as spares for 61, Laser, Crusader & Blaydon Racers.
If it's not Mercs, I haven't a clue where they going - I can't think of any route that require Deckers or a route that needs minibuses that has a PVR of 3/4.
RE: Go North East: 2014/15 Financial Year Order Predictions