Years ago, when boeing was making the 707, 727, and 737 all with the same fuselage diameters, there used to be a joke that Boeing didn't have an aircraft factory but a huge sausage machine that would crank out the fuselage length you wanted, then they'd tack on wings and a tail and send it to you.
I've seen that too, and I can almost believe it.
I built a 707-700 that was two -420s joined end to end, with uprated P&Ws and did it in PanAm colours, entitled 'Clipper Long Tom'. Until you posted that I'd forgotten I'd done it, I'll have to dig the thing out and post some pics. Boeing really DID plan the 707-700 to compete with the DC8-60 series, but it would have needed longer landing gear and thus a new wing root design so it never happened, except in 1/144 scale of course.
How about a 737-50 with maybe 5 windows either side of the wing? Or a loooooong 727-800 with a MONSTER third engine in the centre position? Maybe give it a booster with the standard engine, a la Trident 3?
Of course Airbus are already in the sausage machine business, with the A318, A319, A320 and A321. An A312 would look good, or maybe even an A329???