Great news about the electric buses!

The 57 would certainly seem to fit the bill with working through Central Newcastle & Central Gateshead, & using the Tyne Bridge and Coast Rd which are both pollution hotspots.

However my initial guess was the 53/54 if I'm being honest, as this is a shorter route than runs more centrally and closer to the depot for getting back to the chargers, (although an extra charger could always be put in at Gateshead Interchange for use on layovers?).

Thinking aloud here, I'd re-route 53/54 so it pretty much went around the bus loop in Newcastle City Centre.
(Northbound from Gateshead Interchange it would call at Hills St, then across the Tyne Bridge and serve Tyne Bridge North End, Market St East, John Dobson St/City Hall OR St Mary's Place for Haymarket, possibly Eldon Square Bus Station, then Newgate St/Maldron Hotel, Grainger St South for Central Station, St Nicholas St/High Level Bridge North, Gateshead Wellington St, Gateshead Interchange, and then after that I'm not too sure. Maybe it could go further down Durham Road as that is a busy corridor and it will still serve Saltwell Park half way along it, then terminate & turn back at the Angel of the North/A1 roundabout which in future may be a Park & Ride site by the sounds of it).  

I'd also have it renumbered & rebranded to promote it more as a central shuttle linking the two major central areas together.
A bit like the Quaylink is still often more noticeable and accessible to tourists over other local buses, this too would become a known service to tourists, residents, and even longer distance/park & ride commuters as the shuttle for getting in, around & between Newcastle & Gateshead, and along Durham Road.
"City Shuttle" - The clean green city centre connection machine.    ;-)    
Much better than 53/54 Saltwell Park which I think as a brand doesn't entice anyone. The different numbers for different directions of the circular route is confusing too, along with the 'Newcastle' destination even on Gateshead-bound services. 

...Having said that if it was going to branch out down Durham Rd it could be a cross-city service and branch out down the Coast Rd at the other end of the route, (turning back at the Wallsend roundabout), OR go down the Great North Road as that is polluted too ...but already has a wealth of bus services I suppose. I guess making it cross-city though is almost leaning back towards the Citylink57 suggestion.

Oh well, we'll have to wait and see!