If you were to design a tone arm, what would it look like and feature?

^^ Triplanar seems to keep busy somehow- if anything more busy than they used to be.

Having made a number of recordings, some of which I have mastered to LP myself, IMO the Triplanar is currently the state of the art and has been for some time. But lacking that sort of reference I can easily see why someone might think otherwise.

For tracking geometry and other issues, the Thales Simplicity II/Statement are hard to beat.

The Thales arm is interesting in design; it is a version of the Garrard zero 100 tonearm from the 1970's.  The theoretical disadvantage is having four pivots  for movement in the horizontal plane.  I also don't know if there are advantages or disadvantages to the twin arm tubes (rigidity, inertial mass).  Also, unlike the Reed arm, the Thales does not eliminate skating force because there is an offset angle to its headshell.

I've heard some very nice setups with the Triplanar arm and I certainly think it is in play among top designs.  I also heard a nice setup with a Durand arm.  Durand makes gimbal and uni-pivot arms; their top model is a uni-pivot arm (Telos, a model I have not heard).

An auto arm lifter (gentle) at the end of a record that does not thump the arm

Just a thought and I will take this up with my son in law to be an industrial robotics engineer. Robots have gotten accurate beyond belief with motors and drive systems accurate to less than a thousandth of an inch. It should be possible I would think to watch the groove with an optical sensor and drive a tangential tonearm accordingly. This would be far more robust than whisker contacts. 
He will be home from China in 10 days and we will see if that idea is feasible given current technology.