A physics genre. Not just the kinematics and forces and momentum that is standard with MOST physics engines. I want to see a game that takes physics to the nth degree! After all this time studying electromagnetism, I want to see someone program in the laws of electromagnetism with boundary conditions and the Lorentz force law to so some awesome things like:
Create electric charge to store. If an object is closer to the charge, the charge will jump like lightning. If a conductive material is present, the charge will want to jump there first instead of a nonconductive material.
Deflect plasma shots with magnetic fields. Since plasma shots are essentially charged particles, you could use a magnetic field to deflect the charged particles. OR you could fire another plasma shot parallel but a few inches away from the opposing shot to deflect it using its own magnetic field (because moving charges create magnetic fields).
Create electromagnetic waves to cook enemies! The more powerful you become, the further down the EM spectrum you could go! Radio could be used as a tool, Microwaves could be used as a tool and a weapon, UV, X-Ray, and Gamma could be used as weapons.
Electric fields could also be used to pick up metals and non-conductive media.
ALSO What about relativity and stuff? What if there was a relativistic speed race simulator? Players would have to think about the consequences of time dilation, length dilation, and how fast another person is moving ahead of you. Since length is dilated, things that you've already passed would appear ahead of you. I'm dead serious, because this would be freaking awesome.