Phoneblok, a concept that has appeared online just recently, talked about a phone that is green to the environment and allows us to upgrade the individual parts anytime you like, depending on what is most important to you.
Of course, this is a concept, but it might just be possible given the advancement of electronics. Maybe the engineers need to think about the back-plane of the main PCB, and how the contacts can work together (e.g. using the USB bus concept?), and how information can be transmitted across the back-plane to talk to the CPU, memory, camera, battery, etc. But I like the idea of customising the phone based on the parts that matters most to me.
I would opt for a longer life battery, a better camera, and maybe a high definition screen. What about you?