There are a couple of people I feel Apple would put on their payroll for their visionary aspects with engine design and user interaction.
The first is Ken Silverman. He created Build for Duke Nukem 3D, Shadow Warrior, Blood, and Kens Labyrinth. It is still relevant today especially for mobile devices. Ill show you a comparison between Build's original version and a modernized version.
This is the original Build engine:
Build using Polymer:
The engine holds up well to fairly modern games such as F.E.A.R. It just needs ragdolls, bones, and particles and scripting. Particles, ragdolls, and bones aren't difficult. The fundementals are there(moving doors, destructible environments, slanted doors/ceilings, scripted actions). He did all of this when he was 17. He shouldn't be a programmer. He should be on the payroll to design an in house engine.
The next person...
Warren Specter created Ultima Underworld, System Shock and most importantly Deus Ex. He has pushed first person role playing games. He works for Disney now so it should be easy to bring him in. IMO he is a better fit as a UI lead along side Jony Ive. Warren Specter is great with choices/events and blending UI and actions for what someone intends to do.
Im being an armchair CPO so I hope everyone brings things to the table in a constructive fashion.