Right now, the main thing preventing Boeing from doing a 767 NEO (which is what the A360 pictured above basically is) is the lack of a suitable engine. We're talking ~50k-60k lb. thrust class, with the SFC of GEnx or Trent 1000 (or better). There isn't even anything on the drawing board at the big three engine makers in that thrust class.
Engine development costs what they are, you're talking a couple $Billion just for the engines. Unless someone determines that market for a 757 replacement is a whole lot bigger than most current estimates, it economically just doesn't work. If such an airplane is developed, my expectation is it'll be twin aisle (see my comment above about turn times on 250 passenger single aisle). And if it's not a derivative of a current aircraft, I doubt the fuselage will be 'round'.