Yes, I think Boeing is going to launch a NG767 program anytime soon even if the 757 goes out of production in three years from now. That's what's gonna happen, the 757 will probably go out of production in five years from now and the 767 will continue to be built for many more years to come. Boeing has made a 767-400 already, although this one hasn't received many orders yet a new version is under study, the 400ERX. I would consider the current 400 as being part of the NG767 program.
Although Boeing isn't receiving anymore orders for the 200/200ER, it is certainly still available because the tooling required to build any version of the 767 is the same. They still built the 300ER and now the 400, so the 200ER could still be built.
This is my prediction: Boeing engineers are going to work on a new 767, the 500, which would have the exact lengh and seating layout as the 200 but the new technology involved in designing the 400 would be incorporated. In other words, the 500 would simply be a shortened 400. Think about it, the 767-200 is almost twenty years old, so major airlines like American and Delta should start thinking of a replacement. The 500 which I describe above would be the perfect replacement of the 200. If Boeing starts working on it now, it could roll out of the factory in three or four years from now and first delivery would take place in five years from now. I'm sure American would be interested in it (so would Detla) because they've been operating the 200 on long haul flights for a long time. American was in fact an early customer for the Boeing 767.
I really think the 767 has a long future, so does the 777. Boeing has been planning the 777-100 which would be a shortened 200 but it would have a very long range, close to 10000NM nonstop! So I wouldn't consider that one as a substitute of the 767-400 eventhough they could carry the same number of passengers and/or the same amount of freight. I agree with you when you say that Boeing will come out with a NG767 family whether or not the 777-100X will be built.
On the other side, Airbus is currently planning an A330-100 which could compete with the new 767 I described. The so called A330-100 could be designated as the A329 and it would also be designed to fly over 7000NM nonstop! The A330-100/200 is the replacement of the A310.
Any questions? Please don't hesitate to ask.