FreeGameDev

Another nice feature: terrain/tracks with a different index of grip. For example if my kart is running over a sheet of ice, it should slide and should be more difficult to steer. Over a body of water could be the same thing, the kart could plane and perhaps lose a little bit of grip.

There's already an option for high traction; I don't suppose this would be too hard to implement. But I'm only a novice at C++ and I'm not familiar with the physics engine.

Making a surface slippery is pretty easy. Making a surface slippery in the right way though (challenging but not frustrating) takes a major effort of tweaking, play-testing, more tweaking, etc.
So it may happen but with our efforts on networked multiplayer it is unlikely we will have the time in the near future to make big changes on the physics engine