First is a Harrier with swing wings to fix the technology window of 1969. Sure it might need boron filament structure to keep the weight down. It's fiction so forget what it would cost.

The second would be a B-1 so I could study the active fuselage bending moment suppression by little vanes vs. the stiffness of the 787 carbon fiber structure. I noticed the way the 787 would jerk in yaw flying over the Atlantic in late November of 2016 and thought it was a harsh yaw damper. Definitely I would love to play with this. Exactly what stretch I would want to compare is up in the air.

Transition as mechanized in the F-35B is something that now is for free. All that variable in a V/STOL Supersonic design would puzzle the Millennials who have already decided to use multiple rotors regardless of efficiency. There are still surprises out there like a AH-1 Cobra helicopter having a roll limit of 120 degrees. Or, that flying a Pitts S-2 like an F-15 pulling up to reverse course in extreme turbulence on takeoff can get into a high speed rolling maneuver that breaks the airplane. -5 to +12 G is OK but like a Sabrejet rolling pullouts near VNE are forbidden.