This one might be borderline silly because of its simplicity: a through-hole (front to back) for a tripod screw, in the bottom left or right corner. With that you could fix the phone to tripods or other mounts, or put a keyring or cord through it to carry it around.
1. A replaceable battery
2. The ability to easily install the operating system I want to use, rather than the one the manufacturer wants me to use
3. The ability to repair and upgrade the phone
4. A death ray or a directed EMP