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

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. 

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

My turntable sits within armreach of the main listening chair. The stylus never makes it past the first runout groove and is lifted. But if at the other side of the room......

@daveyf
I’d like to see an "affordable" tonearm that is able to do VTA on the FLY while the record plays ( the only way to do it so you can hear it).......and not lose its settings (VTF)

I believe of all the tonearms mentioned on this thread so far only one has this capability, and it is not the expensive VTA with remote control one that Larry mentioned. 8^0

Atmasphere ^^^ that is SOTA in my book. VTA on the FLY and not lose settings. A designer needs to think that VTF is important enough to make a design that gets over physics.