There is a gaping hole for a new aircraft in the 200-250 passenger with 4-5,000 mile range. Single aisle don't work well, not only are they uncomfortable, it takes a long time to load/unload that many people with a single aisle (I've been in the back of a 757-300 - and it was a solid 10 minutes after they opened the doors before we could even see any movement).
I've long thought Boeing should do a 767 "X" - new wing and engines, updated flight deck and avionics from the 767-2C with the basic 767 fuselage.
Unless they can do something really magical with composite construction for a twin aisle, it'll be had to make a big enough improvement over the 767 fuselage to justify the investment.
Then again, a new design is going to be a compromise and maybe the reason why Boeing built the 757 and 767 in the first place was they decided a compromise wouldn't work.
The 757 was really intended to be a 727 replacement, while the 767 was a 707 replacement. However relatively late in the game they increased the 757 range/payload such that it wasn't that much less than a 767. The 767 came into it's own when they did the -300 with the updated FADEC engines. But it's worth noting that Boeing developed the 757 and 767 at pretty much the same time for about $2 Billion. Now, that was 1980 dollars, but even today that' pencils out to less than $10 billion. Compare that to the estimated $30 Billion that the 787-8 set them back...