I'd invest in time and money to identify where the flows of traffic were at various times of the day, offer a service which stepped away from the traditional and look at how it could flex across the day to suit demand. 

As an example:
Reduced morning peak 21 via core route. 
Offer a '21P' which covered the bulk of the core route, but diverted via Team Valley. 
This balance would change over the course of the day, with the TVTE service reducing in frequency, but the core 21 increasing.

Reversing for the evening peak, with the majority of the PVR switching back towards TVTE. 

The old fashioned system of chucking resource on a concentrated corridor of City Centre services via slowly dying high streets in Chester, Birtley, Low Fell or Gateshead has long gone.
The numbers of people shopping or working in those High Streets have dropped massively. And they certainly don't live on them. I'd hazard a guess they don't want to get on a bus, to stand on a High Street, to then get on another bus to pass through more dead High Streets to then change on to another bus to get to where they're actually working. 

I'd also look at putting the spanner in the hub and spoke model once and for all too. 

But ultimately, whatever routes exist or come about, they need to differ from the network we have now.
It's not working.