Have the phone detected and used in place of a FOB.
There is no reason for everyone to carry around a key fob when your phone can do all the same functions and more. When you approach your Model S, this feature would have the car detect your phone via Bluetooth and presents the handles. The phone, at the same time, would also automatically display the Tesla App allowing you to open the trunk, lower all windows, etc. One less thing to carry in your pocket (and one less thing to lose).
No other car has a feature like this.
The Model 3 and Y offer this feature. For the S/X, the Tesla phone app can unlock and start the car. It is not automatic (i.e. when the phone is nearby) but automatic operation expected in a future release.
I think you'd need a backup for various reasons, like the Model 3 has (and needs, based on owner reports of unreliability of the phone key). But there's no HW in the S/X for an rfid card, so you'd need to have the fob with you anyway.
One potential problem is the range of bluetooth. When I'm in my office, my phone connects to the hands free of my Leaf when my wife pulls up to the building. If it does the same using BT for the locks it would be something I'd not want to use. Same might go for people parking close to their house. (Unfortunately I don't have that "problem".)