I will be kind. You should read more on aviation. None of these here are possible. You cannot increase the payload by 50% stretching the fuselage. You cannot just replace the engines let alone go from turboprops to turbojets without many second and third order effects. A twin WS-20 would not reach the speed of the Y-20 because the Y-15 has straight wings which suck over M0.7. Furthermore, increasing the thrust doesn't increase the payload on its own because the aircraft's structure has to be able to cope with increased stress.Y-20 would move to 100 ton load capacity with in 2020-27. And twin engine WS-20 (or it's derivative) Y-15 would have higher speed than Y-20. It would be better suited for 40-45 ton capacity. It all depends upon the dynamics of circumstances in defense environment.
