If the Tyne Valley Ten did get new buses, it would most likely be ADL E400MMCs. Simple reason being commonality at Hexham with the X84/X85 as well as being good middle ground (not too overkill but not too lightweight).

Only other option I can see depending on what internal and wider GAG cascade opportunites are available is Cobalt & Coast getting replaced with StreetDecks and 6100-6117 (6100 & 01 spare) getting a partial repaint for the Tyne Valley Ten.