The two big 'Hubs' have a connection to Teesside Park, 2 services direct and 8 via the viaduct (15 minute walk) plus several areas of Stockton does via Tees Flex.

So if a new service was to be made, It would likely have to serve a lot of areas or/and improve a service it's competing against.

I'd personally look at extending the 18 to something like Saltburn - Guisborough - Stokesley - Coulby Newham Parkway - Hemlington - Acklam - Teesside Park - Thornaby Station - Stockton.

Opening up a lot of Teesside to Teesside Park, and Stockton to a direct route to Stokesley, Guisborough and Saltburn could prove popular, and saves people swapping over 2/3 buses.