I have it from two very good independent sources that Boeing is close to solidifying the 797 as some sort of NMA project launching very soon. Sounds like it will be a little bigger than the 763 in terms of seating capacity on the lower end, and going up from there. The smaller model may be a little overpowered to give it characteristics similar to the 757 for much better STOL capability.
I would imagine an aircraft with excellent ULH performance in an all-J config for private customers and somewhere around 9,000-11,000 kilometres of range for commercial use depending on configuration. I would also imagine Boeing will be shooting for 4.3-4.5 tonnes (9,500-9,920 lbs) fuel burn per hour to make it competitive and give it an advantage over the 787-8/9.
In terms of engine offerings, GE will probably scale down their GEnx engine and throw in some advancements included in the GE9x, this hypothetical GEnx-3B will probably produce somewhere around 59,000-64,000 lbs of thrust for the product range.
I would say that Boeing will come up with a very compelling product offering to replace the 757/767 range of aircraft. For people concerned about Airbus getting curb-stomped, don’t worry, I’m sure they’re already looking at their options on how to respond. ;)