1. Composite wings on all flight surfaces and wing box, excluding core hull.
2. Core technology found in place on the 787 installed on 777.
3. Performance improvements for 777-X engine types.
4. New aluminum types on the hull. Increase use of titanium.
5. Extended stretch composite wings folding out prior to take-offs so the 777 fits current airport configuration.
These items would use weight reduction technology and fuel economy engineering found on the 787. Not having an all composite hull is offset by a composite, stretch, and folding wing; extending its range, payload, and economy. The secret is in the sauce, and that will be an opus on wing building. Making it very saucy.
Does this compare to an all composite 787 in performance and economy? That is the tricky question. It may not have to depending on the A-350's family, roll out, and performance numbers.
It may come in at a 10-15 % increase of performance over the current model. 12% would get Boeing notice in a big way. Knowing what the 787 did with computer modeling vs. reality is a big technological advance. Boeing has confidence to configure a 777-X on the shop computer and fly it out the door knowing they hit the mark. Plus they could do this quickly which is more critical than dreaming over an eight year cycle. A three year cycle would use the engineering and technical parts bin from the 787 investment, and added value from that former project. That is why a one year wait is not a problem, because what is already moving forward with its surrogate 787 program benchmarks which can roll onto the 777X program. Its not as simple as I make it sound, but lessons learned don't have to be learned over again, and that alone makes a huge difference in the program cycle.
A lot can be done in the meantime before the go ahead trigger is squeezed. The 777X has a few surprises in its bag as Airbus goes its way cloning the 787 project, since they deemed that undo-able in the first place, now they find themselves spending a fortune to do the undo-able. A bridge is being built from the 777-300 to the 777-X, where that bridge can handle a lot of traffic.