I bet the vast majority of any fuel savings will come from the engines; engines that can be designed into to the 737 as a improved 737 fairly easily and cheaply.
A major concern is how reliable COMAC will be as a supplier and, most importantly, supporter of reliable airliners. We have seen many a aerospace company come and go with new technology, but don't last because they are very poor as a supplier. This is something unfortunately, is something one establishes between themselves any any potential customer; the customer has to trust the supplier that the supplier will be reliable and will supply high quality goods. It's pretty hard to establish these relationships, but it is pretty easy to destroy them. And it is almost impossible to reestablish the relationship if it is destroyed.
Not only that, more established competitors may also beat the C-919 to market; Bombardier is working on the C-Series; it will beat the C-919 to market by 2 years; this is a major advantage in the aerospace game. It gives tons of extra time to market and mature the design. Again, I doubt there will be any significant orders outside of China like we are seeing with the ARJ-21.