Obviously, this is all IMHO . . . your mileage may vary, some terms and conditions apply, see disclaimer, you may have certain other legal rights, etc . . .

A clamshell design has its own pros and cons, which depend on usage. For a device that's used as a telephone -- which is how I'd use a successor to the N900 -- the cons predominate.

(By clamshell, I mean something hinged on the long edge like this Pyra or Gameboy DS or the old Sharp Zaurus (the clamshell ones, natch). I don't mean a "flip phone" style device hinged on the shorter side -- that's a different animal.)

The pros I can think of for a clamshell are the protection that it affords the screen when closed, the ability to use the lid state as an on-off or run-pause (or whatever) switch, and the (potential) ability to angle the screen for optimal viewing -- e.g. while it sits on a table and you watch a video.

The cons are mostly about how the form factor dictates that the device can only be used in the "open" state. With a slider like my N900, I can place or receive a call (mostly) one-handed from the touchscreen with the keyboard stowed. A slider or touchscreen-only device doesn't need an auxiliary screen for caller ID.

A clamshell design assumes I'm always gonna be interacting with the unit using the keyboard (or controller pad for a game device.) That makes it perfect for a tiny computer. A better tiny computer than a touchscreen-only smartphone, I'd say. Perhaps a better tiny computer than my N900, but again, at the expense of one-handed usage cases.

For me (and sadly far too few others outside of this forum) the N900 represents a device that provides a great combination of computer and phone features that matches my needs nicely. It replaced my candybar phone, my Palm Pilot, and my SL-5500 -- and it was smaller than two of them! I really want an affordable modern successor. But as they say: "If wishes were horses then beggars would ride." Maybe the solution is selling the kids.