The aerodynamics, he was then a flight simulator modder, and said his dream was to make a game where you could use parts to build an arbitrarily shaped airplane and it would calculate automatically the aerodynamics, because the manual setup of aerodynamics for flight simulator was really bad and annoying.
Granted wasn't him that implemented airplane aerodynamics on KSP (it was a modder of KSP, his mod being considered "mandatory" with how much it changed the game physics)