Tesla has done such a fantastic job on the S, there is not that much room for fruther improvement. Probably best to concentrate on completely new models like the X, III, and the F-150 killer.
However, I will add this list anyway:
Tougher, more durable paint (maybe Tesla's new "world best"paint shop will provide this later this year)
Better cup holders
Regen paddles on steering wheel
Further engineering contineous improvement design changes (better quality and reliability, lower cost)
"100DL" model with stretched wheelbase for spacious rear cabin with more headroom also. (Similar to BMW and Mercedes)
Heated armrests, and maybe door panels and/or headliner (more efficient and comfortable than resistive cabin heater or heat pump)
More software features (autopilot, charge and/or precondition based on departure time, apps, etc.)
GSP