Surely any investment will be focused on the LEZ therefore displacements or swaps to move on:

Streetlites on 35, 52 and 54 (supposedly can't be upgraded).
Scanias on 685 and Berwick Expresses
Volvo B7TL at Blyth, Jesmond and Ashington
Commanders at Blyth
Solos on 46, 51, 55

The 685 has been forgotton about for awhile especially now that Stagecoach runs new deckers on it, could be one for an upgrade. Be very surprised if they mod the Scanias.