All the X39 needs is what is has now but with more reliable allocations.
Most people start work between 8 and 9 and finish between 4 and 6 which is what the 22X and X39 offer. It's not ideal in between having to use the 309 as it takes an age comparatively.
Links to places like Heworth and Gateshead (as the X39 previously did) would be more useful but at the moment the X39 is designed around operational convenience and not passenger growth
More could be made of the 19 to connect at Percy Main and Northumberland Park Metro stations to offer faster connections
RE: Reversing the decline in passenger numbers
Wistfully stuck in the 90s