Foreseen issues:
1. You'd have to totally redo queensboro plaza to achieve those routings efficiently, a not-insignificant task.
2. You'd have to convert all platforms/tracks on the 7 to B division specs, and then reconstruct platforms on the Astoria line to serve A division cars.
3. Unless you swapped the N and R south of Canal (which would lead to yard issues with the N, unless corona got a rebuild), you'd get R trains crossing exp-local at 42nd, obstructing Ns which would be crossing local-exp, causing massive delays.
4. By rerouting R trains away from Queens Plaza, you're eliminating vital transfers for QB riders, making their lives more complicated.