Multi-sim support
What would be really useful for me, and I suspect a lot of others, would be the ability to essentially have two SIMs in the same phone and receive calls on each plus make calls on whichever is appropriate. I have to carry a cruddy work mobile where the SIM is not data enabled and that's what stops me buying a smartphone of any type. If I could have one device that supported my work number and a personal number, allowing me to use the personal for data and personal calls and the work one for work-related calls I'd be on it like a shot. I know you can buy dodgy Chinese phones that support two SIMs and you can get adaptors to hack two SIMs into one, but I'm amazed that none of the major suppliers support this out of the box - if Apple's magic SIM gave me a way to do this I'd buy today.