Shenzhen
I agree - on the face this appears to be an odd route.
I have just returned from a trip to that very city: it is extremely prosperous with a strong propensity for outbound travel, is growing and developing very quickly and is the Chinese version of 'Silicon Valley'.
With a new fast train link to HK, you can be in Kowloon in around 20 minutes from Shenzhen. It could then have potential but.....you don't need a Chinese visa to land into HK whereas you do for Shenzhen (even if you are then going straight to HK on the train) and onward flight connections from Shenzhen are fairly limited apart from domestic routes, we all know that you can get virtually anywhere from HK!
Shenzhen Airport is well designed and I found a breeze to get through, even Immigration, perhaps I was lucky!
But it is still an oddball route. However, I can highly recommend Shenzhen for a visit though (as part of a trip to China) - great food, cheap 5 star hotels and a fair bit to see and do in the vicinity. Climate is very nice - it was 22 degrees when I was there before Christmas! Whether all this is enough for this to be a sustainable routes, let's see, but does it have potential as an alternative entry point to China??