We've kind of min/maxed the design of the smartphone at this point. It's at a physical minimum: a touchscreen and maybe a couple of buttons, but it's also at a utility maximum: you can do almost anything with a phone that you need to use a computer for.

What's next would be what comes after an advanced computer in your pocket. Logically, that must be either a wearable or an implantable device.

At some point, they stopped being phones and became extremely powerful all-purpose computing devices. What comes next?

I just wish alternate form factors of phones had gotten more attention. A plain glass rectangle is absolutely a local minimum of design, but I think it's very far from the best form factor.

For my money, I bet we're stuck with phones until implants become commonplace. There's just not that much else you can do with the 'computer in pocket' form factor

My only other thought is if we discover a reliable method for producing holograms. Then we'll probably see something similar in form to a phone, but which projects a full desktop computer environment, or whatever computing environment is appropriate for the task.