I think one of the biggest advantages of the A321 over the 737-900ER is payload range and the ability to carry cargo containers and pallets, while the 737 and 757 can not.
Its cargo capability, much lower weight and operating costs was one of the factors that pushed the 757 out of the market.
The containers in question... 114 cm high, 156 cm bottom width, 241 cm top width... they cannot fit B737 or B757 solely because they are built specially for A320. Mind yo, they cannot easily be packed into A330 or A380 either - plenty of space wasted.
Which means that unless your Greenliner uses the exact belly hold cross-section of A320, there will be space left over and wasted in Greenliner belly hold. And if you do build Greenliner belly hold to fit the cross-section of A320 belly hold while making the cabin wider, you are getting farther from double bubble, which means your floor loading and weight increases.