JUST to answer the question pithily: the tablet. It’s already here. You’re in the future right now.
So, I guess now the question is, going off of Schiller’s grand philosophical theory (of the Apple product line): what goes between the phone and the tablet? The tablet and the laptop? The laptop and the desktop? What comes after the desktop? What comes before the phone?
 - “Ideally, you should be using the smallest possible gadget to do as much as possible before going to the next largest gizmo in line.”
I think the desktop as a distinct form factor is on its way out. By “desktop” I mean the classic home/office/business multipurpose computer. So I don’t think anything sits between the laptop and the desktop – except maybe an external monitor.
phone => tablet => tablet+keyboard (maybe) =>
laptop => laptop+external-monitor => high-end workstation
Further, I think the table and tablet+keyboard segments will eventually be replaced by phones with a high bandwidth wireless link to a “tablet display”, much like an external monitor but with touch capability – possibly with graphics and cpu on the tablet somehow working together with a phone.
phone => phone+tablet-smart-screen => phone+tablet-smart-screen+keyboard (maybe) =>
phone+laptop-like-dock => phone+externa-monitor+keyboard+mouse =>
I think people generally want fewer systems they have to manager and keep in sync, but I think people are generally ok with having more accessories (like external monitors, TVs, keyboards, mice) that they can keep in different locations (office, home, etc) and which they simply use to change the experience of how they interact with their fewer systems.
PAN hubs + whatever gadgetry users may fancy, freed from the tyranny of uplink handling.